bridge: add proper RCU annotation to should_route_hook

Add br_should_route_hook_t typedef, this is the only way we can
get a clean RCU implementation for function pointer.

Move route_hook to location where it is used.

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>