bonding: use rcu_access_pointer() in bonding_show_mii_status()

curr_active_slave is rcu protected, and bonding_show_mii_status() only
wants to check if pointer is NULL or not.

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>