[VLAN]: nested VLAN: fix lockdep's recursive locking warning

Allow vlans nesting other vlans without lockdep's warnings (max. 2 levels
i.e. parent + child). Thanks to Patrick McHardy for pointing a bug in the
first version of this patch.

Reported-by: Benny Amorsen
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan.c

@@ -323,6 +323,7 @@ static const struct header_ops vlan_header_ops = {
 static int vlan_dev_init(struct net_device *dev)
 {
 	struct net_device *real_dev = VLAN_DEV_INFO(dev)->real_dev;
+	int subclass = 0;
 
 	/* IFF_BROADCAST|IFF_MULTICAST; ??? */
 	dev->flags  = real_dev->flags & ~IFF_UP;
@@ -349,7 +350,11 @@ static int vlan_dev_init(struct net_device *dev)
 		dev->hard_start_xmit = vlan_dev_hard_start_xmit;
 	}
 
-	lockdep_set_class(&dev->_xmit_lock, &vlan_netdev_xmit_lock_key);
+	if (real_dev->priv_flags & IFF_802_1Q_VLAN)
+		subclass = 1;
+
+	lockdep_set_class_and_subclass(&dev->_xmit_lock,
+				&vlan_netdev_xmit_lock_key, subclass);
 	return 0;
 }