switchdev: fix stp update API to work with layered netdevices

make it same as the netdev_switch_port_bridge_setlink/dellink
api (ie traverse lowerdevs to get to the switch port).

removes "WARN_ON(!ops->ndo_switch_parent_id_get)" because
direct bridge ports can be stacked netdevices (like bonds
and team of switch ports) which may not implement this ndo.

v2 to v3:
	- remove changes to bond and team. Bring back the
	transparently following lowerdevs like i initially
	had for setlink/getlink
	(http://www.spinics.net/lists/netdev/msg313436.html)
	dave and scott feldman also seem to prefer it be that
	way and move to non-transparent way of doing things
	if we see a problem down the lane.

v3 to v4:
	- fix ret initialization

v4 to v5:
	- return err on first failure (scott feldman)

v5 to v6:
	- change variable name (err) and initialize to
	-EOPNOTSUPP (scott feldman).
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/switchdev/switchdev.c

@@ -47,11 +47,20 @@ EXPORT_SYMBOL_GPL(netdev_switch_parent_id_get);
 int netdev_switch_port_stp_update(struct net_device *dev, u8 state)
 {
 	const struct swdev_ops *ops = dev->swdev_ops;
+	struct net_device *lower_dev;
+	struct list_head *iter;
+	int err = -EOPNOTSUPP;
 
-	if (!ops || !ops->swdev_port_stp_update)
-		return -EOPNOTSUPP;
-	WARN_ON(!ops->swdev_parent_id_get);
+	if (ops && ops->swdev_port_stp_update)
 		return ops->swdev_port_stp_update(dev, state);
+
+	netdev_for_each_lower_dev(dev, lower_dev, iter) {
+		err = netdev_switch_port_stp_update(lower_dev, state);
+		if (err && err != -EOPNOTSUPP)
+			return err;
+	}
+
+	return err;
 }
 EXPORT_SYMBOL_GPL(netdev_switch_port_stp_update);