bonding: fix bond_3ad_set_carrier() RCU usage

Currently, its usage is just plainly wrong. It first gets a slave under
RCU, and, after releasing the RCU lock, continues to use it - whilst it can
be freed.

Fix this by ensuring that bond_3ad_set_carrier() holds RCU till it uses its
slave (or its agg).

Fixes: be79bd048a ("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

@@ -2327,32 +2327,32 @@ int bond_3ad_set_carrier(struct bonding *bond)
 {
 	struct aggregator *active;
 	struct slave *first_slave;
+	int ret = 1;
 
 	rcu_read_lock();
 	first_slave = bond_first_slave_rcu(bond);
-	rcu_read_unlock();
-	if (!first_slave)
-		return 0;
+	if (!first_slave) {
+		ret = 0;
+		goto out;
+	}
 	active = __get_active_agg(&(SLAVE_AD_INFO(first_slave).aggregator));
 	if (active) {
 		/* are enough slaves available to consider link up? */
 		if (active->num_of_ports < bond->params.min_links) {
 			if (netif_carrier_ok(bond->dev)) {
 				netif_carrier_off(bond->dev);
-				return 1;
+				goto out;
 			}
 		} else if (!netif_carrier_ok(bond->dev)) {
 			netif_carrier_on(bond->dev);
-			return 1;
+			goto out;
 		}
-		return 0;
-	}
-
-	if (netif_carrier_ok(bond->dev)) {
+	} else if (netif_carrier_ok(bond->dev)) {
 		netif_carrier_off(bond->dev);
-		return 1;
 	}
-	return 0;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 /**