bonding: add proper __rcu annotation for current_arp_slave

Using __rcu annotation actually helps to spot all accesses to
bond->current_arp_slave are correctly protected, with LOCKDEP support.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>