dlci: acquire rtnl_lock before calling __dev_get_by_name()

Otherwise the net device returned can be freed at anytime.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wan/dlci.c

@@ -385,20 +385,24 @@ static int dlci_del(struct dlci_add *dlci)
 	struct net_device	*master, *slave;
 	int			err;
 
+	rtnl_lock();
+
 	/* validate slave device */
 	master = __dev_get_by_name(&init_net, dlci->devname);
-	if (!master)
-		return -ENODEV;
+	if (!master) {
+		err = -ENODEV;
+		goto out;
+	}
 
 	if (netif_running(master)) {
-		return -EBUSY;
+		err = -EBUSY;
+		goto out;
 	}
 
 	dlp = netdev_priv(master);
 	slave = dlp->slave;
 	flp = netdev_priv(slave);
 
-	rtnl_lock();
 	err = (*flp->deassoc)(slave, master);
 	if (!err) {
 		list_del(&dlp->list);
@@ -407,8 +411,8 @@ static int dlci_del(struct dlci_add *dlci)
 
 		dev_put(slave);
 	}
+out:
 	rtnl_unlock();
-
 	return err;
 }