ifnet: if_vmove_reclaim(): Be more robust to check the src / dst vnet

[zlei]

Summary

The ifp is only weak referenced, if_vmove_reclaim() may race with other
threads' if_vmove() or if_detach(), hence ifp->if_vnet is not safe nor
stable to access. Given we know the source and destination prison/vnet,
check them instead.

MFC after: 2 weeks

[zlei]

sys/net/if.c

1286

This assert is wrong. Post fix to D55997.