bonding: fix __get_first_agg RCU usage

Currently, the RCU read lock usage is just wrong - it gets the slave struct
under RCU and continues to use it when RCU lock is released.

However, it's still safe to do this cause we didn't need the
rcu_read_lock() initially - all of the __get_first_agg() callers are either
holding RCU read lock or the RTNL lock, so that we can't sync while in it.

Fixes: be79bd0 ("bonding: add RCU for bond_3ad_state_machine_handler()")
CC: dingtianhong@huawei.com
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_3ad.c

@@ -143,21 +143,24 @@ static inline struct bonding *__get_bond_by_port(struct port *port)
  *
  * Return the aggregator of the first slave in @bond, or %NULL if it can't be
  * found.
+ * The caller must hold RCU or RTNL lock.
  */
 static inline struct aggregator *__get_first_agg(struct port *port)
 {
 	struct bonding *bond = __get_bond_by_port(port);
 	struct slave *first_slave;
+	struct aggregator *agg;
 
 	/* If there's no bond for this port, or bond has no slaves */
 	if (bond == NULL)
 		return NULL;
 
 	rcu_read_lock();
 	first_slave = bond_first_slave_rcu(bond);
+	agg = first_slave ? &(SLAVE_AD_INFO(first_slave).aggregator) : NULL;
 	rcu_read_unlock();
 
-	return first_slave ? &(SLAVE_AD_INFO(first_slave).aggregator) : NULL;
+	return agg;
 }
 
 /**