vlan: make vlan_dev_real_dev work over stacked vlans

Sometimes we might have stacked vlans on top of each other, and we're
interested in the first non-vlan real device on the path, so transform
vlan_dev_real_dev to go over the stacked vlans and extract the first
non-vlan device.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan_core.c

@@ -91,7 +91,12 @@ EXPORT_SYMBOL(__vlan_find_dev_deep);
 
 struct net_device *vlan_dev_real_dev(const struct net_device *dev)
 {
-	return vlan_dev_priv(dev)->real_dev;
+	struct net_device *ret = vlan_dev_priv(dev)->real_dev;
+
+	while (is_vlan_dev(ret))
+		ret = vlan_dev_priv(ret)->real_dev;
+
+	return ret;
 }
 EXPORT_SYMBOL(vlan_dev_real_dev);