udp_tunnel: Fix end of loop test in udp_tunnel_nic_unregister()

This test is checking if we exited the list via break or not.  However
if it did not exit via a break then "node" does not point to a valid
udp_tunnel_nic_shared_node struct.  It will work because of the way
the structs are laid out it's the equivalent of
"if (info->shared->udp_tunnel_nic_info != dev)" which will always be
true, but it's not the right way to test.

Fixes: 74cc6d18 ("udp_tunnel: add the ability to share port tables")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp_tunnel_nic.c

@@ -846,7 +846,7 @@ udp_tunnel_nic_unregister(struct net_device *dev, struct udp_tunnel_nic *utn)
 		list_for_each_entry(node, &info->shared->devices, list)
 			if (node->dev == dev)
 				break;
-		if (node->dev != dev)
+		if (list_entry_is_head(node, &info->shared->devices, list))
 			return;
 
 		list_del(&node->list);