Description of problem: Prior to 4.21 release it was possible to apply to a pod a  sriovnetwork configured with mtu 500 with a IPv6 address.  It still remains possible with the latest version of 4.20.8 to deploy a pod with a sriovnetwork with MTU 500 on an interface with dual stack IPs or just IPv6.

Version-Release number of selected component (if applicable): 4.21

How reproducible: Easeily

Steps to Reproduce:
1. Create a sriovnetworknodepolicy
2. Create a srivnetwork with mtu 500
3. Apply sriovnetwork to pod with IPv6 address on the interface with the sriovnetwork.
Actual results: Pod

Pod fails to deploy

 error adding container to network "sriov-net-samepf-mtu500": failed to enable keep_addr_on_down for interface "net1": open /proc/sys/net/ipv6/conf/net1/keep_addr_on_down: no such file or directory

Expected results:

Pod to deploy

Additional info:

IPv6 requires every link to support a minimum MTU of 1280 bytes, as defined in RFC 8200. Unlike IPv4, IPv6 routers do not fragment packets, so endpoints rely on Path MTU Discovery and ICMPv6 Packet Too Big messages to send packets that fit within the path MTU.