Bluetooth: 6lowpan: Fix lockdep splats

When a device ndo_start_xmit() calls again dev_queue_xmit(),
lockdep can complain because dev_queue_xmit() is re-entered and the
spinlocks protecting tx queues share a common lockdep class.

Same issue was fixed for ieee802154 in commit "20e7c4e8"
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -662,7 +662,26 @@ static netdev_tx_t bt_xmit(struct sk_buff *skb, struct net_device *netdev)
 	return err < 0 ? NET_XMIT_DROP : err;
 }
 
+static struct lock_class_key bt_tx_busylock;
+static struct lock_class_key bt_netdev_xmit_lock_key;
+
+static void bt_set_lockdep_class_one(struct net_device *dev,
+				     struct netdev_queue *txq,
+				     void *_unused)
+{
+	lockdep_set_class(&txq->_xmit_lock, &bt_netdev_xmit_lock_key);
+}
+
+static int bt_dev_init(struct net_device *dev)
+{
+	netdev_for_each_tx_queue(dev, bt_set_lockdep_class_one, NULL);
+	dev->qdisc_tx_busylock = &bt_tx_busylock;
+
+	return 0;
+}
+
 static const struct net_device_ops netdev_ops = {
+	.ndo_init		= bt_dev_init,
 	.ndo_start_xmit		= bt_xmit,
 };