When we remove an interface it is first removed from the interface list
V_ifnet (by if_unlink_ifnet()) and marked as IFF_DYING. We then wait for
any possible references to stop being used (i.e.
epoch_wait/epoch_drain_callbacks) before we tear it fully down.
However, the index in ifindex_table is not removed, so m_rcvif_restore()
can still find the (now dying) interface.
This results in panics, for example when dummynet restores the rcvif
pointer and passes a packet to ip6_input() we can panic because the
AF_INET6 domain has already been removed (so we end up dereferencing a
NULL pointer there).
Check that the interface is not dying before we restore it, which is
equivalent to checking its presence in V_ifnet, and thus ensures that
future accesses (while in NET_EPOCH) are safe.
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