bridge: Fix STP-related panic

After r345180 we need to have the appropriate vnet context set to delete an
rtnode in bridge_rtnode_destroy().
That's usually the case, but not when it's called by the STP code (through
bstp_notify_rtage()).

We have to set the vnet context in bridge_rtable_expire() just as we do in the
other STP callback bridge_state_change().

Reviewed by: kevans