bonding: use rcu_dereference() to access curr_active_slave

bonding: use rcu_dereference() to access curr_active_slave

The bond_info_show_master already in RCU read-side critical section,
and the we access curr_active_slave without the curr_slave_lock, we
could not sure whether the curr_active_slave will be changed during
the processing, so use RCU to protected the pointer.

Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>