If vnets are torn down while ifconfig runs an ioctl to say, destroy an
epair(4), we may hit if_detach_internal() without holding a lock and by
the time we aquire it the interface might be gone.
We should not panic() in this case as it is our fault for not holding
the lock all the way. It is not ideal to return silently without error
to user space, but other callers will all ignore the return values so
do not change the entire KPI for little benefit for now.
The ifp will be dealt with one way or another still.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: gnn
Differential Revision: https://reviews.freebsd.org/D4529