bonding: remove bond read lock for bond_3ad_state_machine_handler()

The bond slave list may change when the monitor is running, the slave list is no longer protected by bond->lock, only protected by rtnl lock(), so we has 3 way to modify it:
1.add bond_master_upper_dev_link() and bond_upper_dev_unlink() in bond->lock, but it is unsafe to call call_netdevice_notifiers() in write lock.
2.remove unused bond->lock for monitor function, only use the exist rtnl lock(). 3.use rcu_read_lock() to protect it, of course, it will transform bond_for_each_slave to bond_for_each_slave_rcu() and performance is better, but in slow path, it is ignored. so I remove the bond->lock and move the rtnl lock to protect the whole monitor function.

---
 drivers/net/bonding/bond_3ad.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

index 187b1b7..d6fe00b 100644
@@ -2068,8 +2068,10 @@ void bond_3ad_state_machine_handler(struct work_struct *work)
 	struct slave *slave;
 	struct port *port;
 
-	read_lock(&bond->lock);
-
+	if (!rtnl_trylock()) {
+		queue_delayed_work(bond->wq, &bond->ad_work, ad_delta_in_ticks);
+		return;
+	}
 	//check if there are any slaves
 	if (!bond_has_slaves(bond))
 		goto re_arm;

@@ -2122,9 +2124,8 @@ void bond_3ad_state_machine_handler(struct work_struct *work)
 	}
 
 re_arm:
+	rtnl_unlock();
 	queue_delayed_work(bond->wq, &bond->ad_work, ad_delta_in_ticks);
-
-	read_unlock(&bond->lock);
 }
 
 /**

1.8.2.1