if_bridge - Fix issue where source MAC is not recorded.

if_bridge - Fix issue where source MAC is not recorded.

* The bridge has to record which interface the source MAC came from
  so transmissions back to that MAC can be unicast.  Otherwise transmissions
  will be broadcast to all bridge elements.

  The receive packet path case where a packet is sent to the bridge's own
  MAC address (terminating on the bridge host) from one of the member
  interfaces was not recording the MAC, resulting in duplicate packets
  (most obvious when testing with 'ping').

* Fix the code path in question so the MAC is properly recorded.