Work around a race condition on interface move between vnets.

[bz]

Summary

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.