Revert "l2tp: constify inet6_protocol structures"

This reverts commit d04916a4.

inet6_add_protocol and inet6_del_protocol include casts that remove the
effect of the const annotation on their parameter, leading to possible
runtime crashes.
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NDavid 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 const struct inet6_protocol l2tp_ip6_protocol = {
+static struct inet6_protocol l2tp_ip6_protocol __read_mostly = {
 	.handler	= l2tp_ip6_recv,
 };