bridge: Remove members when assigned to a new vnet

When the bridge is moved to a different vnet we must remove all of its
member interfaces (and span interfaces), because we don't know if those
will be moved along with it. We don't want to hold references to
interfaces not in our vnet.

Reviewed by: donner@
MFC after: 1 week
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D28859

(cherry picked from commit 38c0951386d82f4c51cf4e245253cdef18d2254a)