When we terminate a vnet (i.e. jail) we move interfaces back to their
home vnet. We need to protect our access to the V_ifnet CK_LIST.
We could enter NET_EPOCH, but if_detach_internal() (called from
if_vmove()) waits for net epoch callback completion. That's not possible
from NET_EPOCH. Instead, we take the IFNET_WLOCK, build a list of the
interfaces that need to move and, once we've released the lock, move
them back to their home vnet.
We cannot hold the IFNET_WLOCK() during if_vmove(), because that results
in a LOR between ifnet_sx, in_multi_sx and iflib ctx lock.
Separate out moving the ifp into or out of V_ifnet, so we can hold the
lock as we do the list manipulation, but do not hold it as we
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