l2tp: constify inet6_protocol structures

The inet6_protocol structure is only passed as the first argument to
inet6_add_protocol or inet6_del_protocol, both of which are declared as
const.  Thus the inet6_protocol structure itself can be const.

Also drop __read_mostly on the newly const structure.

Done with the help of Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip6.c

@@ -788,7 +788,7 @@ static struct inet_protosw l2tp_ip6_protosw = {
 	.ops		= &l2tp_ip6_ops,
 };
 
-static struct inet6_protocol l2tp_ip6_protocol __read_mostly = {
+static const struct inet6_protocol l2tp_ip6_protocol = {
 	.handler	= l2tp_ip6_recv,
 };