MFC r238309:

MFC r238309:

In epair_clone_destroy(), when destroying the second half, we have to
switch to its vnet before calling ether_ifdetach(). Otherwise if the
second half resides in a different vnet, if_detach() silently fails
leaving a stale pointer in V_ifnet list, and the system crashes trying
to access this pointer later.

Another solution could be not to allow to destroy epair unless both
ends are in the home vnet.

Discussed with:	bz
Tested by:	delphij

git-svn-id: svn://svn.freebsd.org/base/stable/9@239980 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f