bridge: add RCU annotations to bridge port lookup

br_port_get() renamed to br_port_get_rtnl() to make clear RTNL is held.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>