MFC r292604:

MFC r292604:

 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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@294502 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f