netfilter: conntrack: gre: convert rwlock to rcu

We can use gre.  Lock is only needed when a new expectation is added.

In case a single spinlock proves to be problematic we can either add one
per netns or use an array of locks combined with net_hash_mix() or similar
to pick the 'correct' one.

But given this is only needed for an expectation rather than per packet
a single one should be ok.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/nf_conntrack_proto_gre.h

@@ -19,6 +19,7 @@ struct nf_conn;
 struct nf_ct_gre_keymap {
 	struct list_head list;
 	struct nf_conntrack_tuple tuple;
+	struct rcu_head rcu;
 };
 
 enum grep_conntrack {