bonding: add proper __rcu annotation for curr_active_slave

RCU was added to bonding in linux-3.12 but lacked proper sparse annotations.

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

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