If the spanning tree root interface is removed from the bridge we panic
on the next 'ifconfig'.
While the STP code is notified whenever a bridge member interface is
removed from the bridge it does not clear the bs_root_port. This means
bs_root_port can still point at an bridge_iflist which has been free()d.
The next access to it will panic.
Explicitly check if the interface we're removing in bstp_destroy() is
the root, and if so re-assign the roles, which clears bs_root_port.
Reviewed by: philip
MFC after: 2 weeks
d6747eafa9