kernel - fix statistics counters for if_bridge.

kernel - fix statistics counters for if_bridge.

* Count input statistics in bridge_input() instead of bridge_forward()

* Add output statistics in bridge_forward().  That is, the bridge should
  in abstract look like a piece of hardware from a statistics standpoint,
  so when a packet is forwarded through a bridge interface it needs to
  show up as both an input packet and an output packet.

* Fixes statistics reporting for e.g. 'netstat -in 1'.