bridge: is PACKET_LOOPBACK unlikely()?

While looking at using netdev_rx_handler_register for openvswitch Jesse
Gross suggested that an unlikely() might be worthwhile in that code.
I'm interested to see if its appropriate for the bridge code.

Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>