Bluetooth: 6lowpan: Memory leak as the skb is not freed

The earlier multicast commit 36b3dd250d ("Bluetooth: 6lowpan:
Ensure header compression does not corrupt IPv6 header") lost one
skb free which then caused memory leak.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

net/bluetooth/6lowpan.c

@@ -625,6 +625,8 @@ static netdev_tx_t bt_xmit(struct sk_buff *skb, struct net_device *netdev)
 		send_mcast_pkt(skb, netdev);
 	}
 
+	dev_kfree_skb(skb);
+
 	if (err)
 		BT_DBG("ERROR: xmit failed (%d)", err);