net: dev: Fix non-RCU based lower dev walker

netdev_walk_all_lower_dev is not properly walking the lower device
list.  Commit 1a3f060c1a47 made netdev_walk_all_lower_dev similar
to netdev_walk_all_upper_dev_rcu and netdev_walk_all_lower_dev_rcu
but failed to update its netdev_next_lower_dev iterator. This patch
fixes that.

Fixes: 1a3f060c1a47 ("net: Introduce new api for walking upper and
                     lower devices")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5400,12 +5400,12 @@ static struct net_device *netdev_next_lower_dev(struct net_device *dev,
 {
 	struct netdev_adjacent *lower;
 
-	lower = list_entry(*iter, struct netdev_adjacent, list);
+	lower = list_entry((*iter)->next, struct netdev_adjacent, list);
 
 	if (&lower->list == &dev->adj_list.lower)
 		return NULL;
 
-	*iter = lower->list.next;
+	*iter = &lower->list;
 
 	return lower->dev;
 }