vgsplit: fix check for not splitting an LV between two VGs for cachevol

When cachevol was used for the cache LV, the check for not splitting an
LV between two VGs was incorrect, not allowing to split the VG in cases
it should have been possible.

For example, splitting a VG which contains the cache LV with cachevol
only on two devices (sda and sdb here) and leaving third device completely
unused (sdc here) should clearly allow us to split it into a new VG:

❯ vgcreate vg /dev/sd

{a..c}
Physical volume "/dev/sda" successfully created.
Physical volume "/dev/sdb" successfully created.
Physical volume "/dev/sdc" successfully created.
Volume group "vg" successfully created

❯ lvcreate -l2 -n main vg /dev/sda
Logical volume "main" created.

❯ lvcreate -l2 -n fast vg /dev/sdb
Logical volume "fast" created.

❯ lvconvert -y --type cache --cachevol fast vg/main
Logical volume vg/main is now cached.

❯ lvs -a -o name,devices vg
lv_name devices
[fast_cvol] /dev/sdb(0)
main main_corig(0)
[main_corig] /dev/sda(0)

❯ lsblk -o name /dev/sd{a..c}

NAME
sda
└─vg-main_corig
└─vg-main
sdb
└─vg-fast_cvol
├─vg-fast_cvol-cdata
│ └─vg-main
└─vg-fast_cvol-cmeta
└─vg-main
sdc

Before this patch:

❯ vgsplit vg vg2 /dev/sdc
Logical volume vg/main must be inactive.

❯ vgchange -an vg
0 logical volume(s) in volume group "vg" now active

❯ vgsplit vg vg2 /dev/sdc
Can't split LV main between two Volume Groups

With this patch applied:

❯ vgsplit vg vg2 /dev/sdc
New volume group "vg2" successfully split from "vg"