Outbound VLAN filtering wasn't being done for host-originated frames,
because bridge_output was missing a call to bridge_vfilter_out, like
in bridge_forward and bridge_broadcast.
Rather than adding another call, move the filtering to bridge_enqueue,
which ensures all frames will be filtered. This slightly changes the
observable behaviour since we now do pfil before vlan filtering, but
that's probably closer to what users expect anyway.