net/802: add __rcu annotations

(struct net_device)->garp_port is rcu protected :
(struct garp_port)->applicants is rcu protected :

add __rcu annotation and proper rcu primitives.

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