net/smc: Fix NULL pointer dereferencing in smc_vlan_by_tcpsk()

Coverity reports a possible NULL dereferencing problem:

in smc_vlan_by_tcpsk():
6. returned_null: netdev_lower_get_next returns NULL (checked 29 out of 30 times).
7. var_assigned: Assigning: ndev = NULL return value from netdev_lower_get_next.
1623                ndev = (struct net_device *)netdev_lower_get_next(ndev, &lower);
CID 1468509 (#1 of 1): Dereference null return value (NULL_RETURNS)
8. dereference: Dereferencing a pointer that might be NULL ndev when calling is_vlan_dev.
1624                if (is_vlan_dev(ndev)) {

Remove the manual implementation and use netdev_walk_all_lower_dev() to
iterate over the lower devices. While on it remove an obsolete function
parameter comment.

Fixes: cb9d43f6 ("net/smc: determine vlan_id of stacked net_device")
Suggested-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/smc/smc_core.c

@@ -1672,14 +1672,26 @@ static void smc_link_down_work(struct work_struct *work)
 	mutex_unlock(&lgr->llc_conf_mutex);
 }
 
-/* Determine vlan of internal TCP socket.
- * @vlan_id: address to store the determined vlan id into
- */
+static int smc_vlan_by_tcpsk_walk(struct net_device *lower_dev,
+				  struct netdev_nested_priv *priv)
+{
+	unsigned short *vlan_id = (unsigned short *)priv->data;
+
+	if (is_vlan_dev(lower_dev)) {
+		*vlan_id = vlan_dev_vlan_id(lower_dev);
+		return 1;
+	}
+
+	return 0;
+}
+
+/* Determine vlan of internal TCP socket. */
 int smc_vlan_by_tcpsk(struct socket *clcsock, struct smc_init_info *ini)
 {
 	struct dst_entry *dst = sk_dst_get(clcsock->sk);
+	struct netdev_nested_priv priv;
 	struct net_device *ndev;
-	int i, nest_lvl, rc = 0;
+	int rc = 0;
 
 	ini->vlan_id = 0;
 	if (!dst) {
@@ -1697,20 +1709,9 @@ int smc_vlan_by_tcpsk(struct socket *clcsock, struct smc_init_info *ini)
 		goto out_rel;
 	}
 
+	priv.data = (void *)&ini->vlan_id;
 	rtnl_lock();
-	nest_lvl = ndev->lower_level;
-	for (i = 0; i < nest_lvl; i++) {
-		struct list_head *lower = &ndev->adj_list.lower;
-
-		if (list_empty(lower))
-			break;
-		lower = lower->next;
-		ndev = (struct net_device *)netdev_lower_get_next(ndev, &lower);
-		if (is_vlan_dev(ndev)) {
-			ini->vlan_id = vlan_dev_vlan_id(ndev);
-			break;
-		}
-	}
+	netdev_walk_all_lower_dev(ndev, smc_vlan_by_tcpsk_walk, &priv);
 	rtnl_unlock();
 
 out_rel: