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

netdev_walk_all_lower_dev is not properly walking the lower device
list.  Commit 1a3f060c 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: 1a3f060c ("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>