bonding: fix locking in enslave failure path

In commit 3c5913b5 ("bonding:
primary_slave & curr_active_slave are not cleaned on enslave failure")
I didn't account for the use of curr_active_slave without curr_slave_lock
and since there are such users, we should hold bond->lock for writing while
setting it to NULL (in the NULL case we don't need the curr_slave_lock).
Keeping the bond lock as to avoid the extra release/acquire cycle.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1915,14 +1915,16 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 	bond_detach_slave(bond, new_slave);
 	if (bond->primary_slave == new_slave)
 		bond->primary_slave = NULL;
-	write_unlock_bh(&bond->lock);
 	if (bond->curr_active_slave == new_slave) {
+		bond_change_active_slave(bond, NULL);
+		write_unlock_bh(&bond->lock);
 		read_lock(&bond->lock);
 		write_lock_bh(&bond->curr_slave_lock);
-		bond_change_active_slave(bond, NULL);
 		bond_select_active_slave(bond);
 		write_unlock_bh(&bond->curr_slave_lock);
 		read_unlock(&bond->lock);
+	} else {
+		write_unlock_bh(&bond->lock);
 	}
 	slave_disable_netpoll(new_slave);