When we shut down a vnet jail its interfaces get moved back into their
original vnet. If we simultaneously delete that interface there's a
possible race where the interface is being moves while it's being
destroyed. That can result in if_vmove() attaching the interface into
the original vnet, and immediately freed. We then have a freed ifp in
V_ifnet, which will panic the next time we end up accessing it.
Don't re-attach dying interfaces.
This is frequently triggered by the automated pf tests.