Description of problem:
packet replied for load balance is not snated if both lb and nat are added

Version-Release number of selected component (if applicable):
ovn-2021-20.12.0-20

How reproducible:
Always

Steps to Reproduce:
1. setup on hv1:

systemctl start openvswitch
systemctl start ovn-northd
ovn-nbctl set-connection ptcp:6641
ovn-sbctl set-connection ptcp:6642
ovs-vsctl set open . external_ids:system-id=hv1 external_ids:ovn-remote=tcp:20.0.180.25:6642 external_ids:ovn-encap-type=geneve external_ids:ovn-encap-ip=20.0.180.25
systemctl restart ovn-controller

ovs-vsctl add-br br-ex
ovs-vsctl set open . external_ids:ovn-bridge-mappings=provider:br-ex
ovs-vsctl add-port br-ex ens1f1
ip link set ens1f1 up
ip link set br-ex up

ovn-nbctl ls-add ls1
ovn-nbctl lsp-add ls1 ls1p1
ovn-nbctl lsp-set-addresses ls1p1 "00:00:00:01:01:11 192.168.1.11 2001::11"
ovn-nbctl lsp-add ls1 ls1p2
ovn-nbctl lsp-set-addresses ls1p2 "00:00:00:01:01:12 192.168.1.12 2001::12"
ovn-nbctl lsp-add ls1 ls1p3
ovn-nbctl lsp-set-addresses ls1p3 "00:00:00:01:01:13 192.168.1.13 2001::13"
ovn-nbctl lr-add lr1
ovn-nbctl lrp-add lr1 lr1-ls1 00:00:00:00:00:01 192.168.1.254/24 2001::a/64
ovn-nbctl lsp-add ls1 ls1-lr1
ovn-nbctl lsp-set-addresses ls1-lr1 router
ovn-nbctl lsp-set-type ls1-lr1 router
ovn-nbctl lsp-set-options ls1-lr1 router-port=lr1-ls1

ovn-nbctl ls-add ls_pub
ovn-nbctl lsp-add ls_pub ls_pub_ln
ovn-nbctl lsp-set-options ls_pub_ln network_name=provider
ovn-nbctl lsp-set-type ls_pub_ln localnet
ovn-nbctl lsp-set-addresses ls_pub_ln unknown

ovn-nbctl lrp-add lr1 r1-ln 00:00:00:00:00:03 172.18.1.254/24 7011:18::1/64
ovn-nbctl lrp-set-gateway-chassis r1-ln hv0
ovn-nbctl lsp-add ls_pub ls_pub-r1
ovn-nbctl lsp-set-type ls_pub-r1 router
ovn-nbctl lsp-set-addresses ls_pub-r1 router
ovn-nbctl lsp-set-options ls_pub-r1 router-port=r1-ln

ovn-nbctl lr-nat-add lr1 dnat_and_snat 172.18.1.11 192.168.1.11 ls1p1 00:00:00:00:01:11
ovn-nbctl lr-nat-add lr1 dnat_and_snat 172.18.1.12 192.168.1.12 ls1p2 00:00:00:00:01:12

ovn-nbctl lb-add lb_tcp 172.18.1.50:50001 192.168.1.11:50001,192.168.1.12:50001,192.168.1.13:50001 tcp
ovn-nbctl lr-lb-add lr1 lb_tcp
ovn-nbctl set load_balancer lb_tcp selection_fields="ip_src,ip_dst,tp_src,tp_dst"

ip netns add ls1p1
ovs-vsctl add-port br-int ls1p1 – set interface ls1p1 type=internal external_ids:iface-id=ls1p1
ip link set ls1p1 netns ls1p1
ip netns exec ls1p1 ip link set ls1p1 address 00:00:00:01:01:11
ip netns exec ls1p1 ip link set ls1p1 up
ip netns exec ls1p1 ip addr add 192.168.1.11/24 dev ls1p1
ip netns exec ls1p1 ip route add default via 192.168.1.254 dev ls1p1
ip netns exec ls1p1 nc -k -l 50001 &

ip netns add ls1p2
ovs-vsctl add-port br-int ls1p2 – set interface ls1p2 type=internal external_ids:iface-id=ls1p2
ip link set ls1p2 netns ls1p2
ip netns exec ls1p2 ip link set ls1p2 address 00:00:00:01:01:12
ip netns exec ls1p2 ip link set ls1p2 up
ip netns exec ls1p2 ip addr add 192.168.1.12/24 dev ls1p2
ip netns exec ls1p2 ip route add default via 192.168.1.254 dev ls1p2
ip netns exec ls1p2 nc -k -l 50001 &

ovs-vsctl add-port br-ex ext1 – set interface ext1 type=internal
ip netns add ext1
ip link set ext1 netns ext1
ip netns exec ext1 ip link set ext1 up
ip netns exec ext1 ip addr add 172.18.1.1/24 dev ext1

2. setup on hv0

systemctl start openvswitch
ovs-vsctl set open . external_ids:system-id=hv0 external_ids:ovn-remote=tcp:20.0.180.25:6642 external_ids:ovn-encap-type=geneve external_ids:ovn-encap-ip=20.0.180.26
systemctl restart ovn-controller

ovs-vsctl add-br br-ex
ovs-vsctl set open . external_ids:ovn-bridge-mappings=provider:br-ex
ovs-vsctl add-port br-ex ens1f1
ip link set ens1f1 up
ip link set br-ex up

ip netns add ls1p3
ovs-vsctl add-port br-int ls1p3 – set interface ls1p3 type=internal external_ids:iface-id=ls1p3
ip link set ls1p3 netns ls1p3
ip netns exec ls1p3 ip link set ls1p3 address 00:00:00:01:01:13
ip netns exec ls1p3 ip link set ls1p3 up
ip netns exec ls1p3 ip addr add 192.168.1.13/24 dev ls1p3
ip netns exec ls1p3 ip route add default via 192.168.1.254 dev ls1p3
ip netns exec ls1p3 nc -k -l 50001 &

ip netns add ext2
ovs-vsctl add-port br-ex ext2 – set interface ext2 type=internal
ip link set ext2 netns ext2
ip netns exec ext2 ip link set ext2 up
ip netns exec ext2 ip addr add 172.18.1.2/24 dev ext2

3. run nc on hv1:

for i in

; do
ip netns exec ext1 nc 172.18.1.50 50001 <<< h; done

Actual results:
part of the nc would fail with time out
Ncat: Connection timed out.

Expected results:
all nc passed

Additional info:

from the tcpdump on ext1, I see that if the lb ip is load balanced to 192.168.1.11 or 192.168.1.12 which locate on hv1, the replied packet to ext1 is not snated:

02:13:42.275182 4a:cf:0c:a6:27:d0 > 00:00:00:00:00:03, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 48738, offset 0, flags [DF], proto TCP (6), length 60)
172.18.1.1.46948 > 172.18.1.50.50001: Flags [S], cksum 0x5a86 (incorrect -> 0x45e2), seq 3994669128, win 29200, options [mss 1460,sackOK,TS val 2174974713 ecr 0,nop,wscale 7], length 0
02:13:42.276894 00:00:00:00:01:12 > 4a:cf:0c:a6:27:d0, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 63, id 0, offset 0, flags [DF], proto TCP (6), length 60)
192.168.1.12.50001 > 172.18.1.1.46948: Flags [S.], cksum 0x6ef6 (incorrect -> 0x136c), seq 407500889, ack 3994669129, win 28960, options [mss 1460,sackOK,TS val 1810276955 ecr 2174974713,nop,wscale 7]
, length 0

02:13:52.322551 4a:cf:0c:a6:27:d0 > 00:00:00:00:00:03, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 44848, offset 0, flags [DF], proto TCP (6), length 60)
172.18.1.1.46950 > 172.18.1.50.50001: Flags [S], cksum 0x5a86 (incorrect -> 0x5802), seq 3894254818, win 29200, options [mss 1460,sackOK,TS val 2174984761 ecr 0,nop,wscale 7], length 0
02:13:52.324299 00:00:00:00:01:11 > 4a:cf:0c:a6:27:d0, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 63, id 0, offset 0, flags [DF], proto TCP (6), length 60)
192.168.1.11.50001 > 172.18.1.1.46950: Flags [S.], cksum 0x6ef5 (incorrect -> 0xdbfd), seq 1741590943, ack 3894254819, win 28960, options [mss 1460,sackOK,TS val 2400837612 ecr 2174984761,nop,wscale 7
], length 0