ieee802154: 6lowpan: add RTNL assertion

This patch ensure that the rtnl lock is hold while newlink callback.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan_rtnl.c

@@ -584,6 +584,8 @@ static int lowpan_newlink(struct net *src_net, struct net_device *dev,
 	struct lowpan_dev_record *entry;
 	int ret;
 
+	ASSERT_RTNL();
+
 	pr_debug("adding new link\n");
 
 	if (!tb[IFLA_LINK])