netfilter: fix nf_conntrack_l4proto_register()

While doing __rcu annotations work on net/netfilter I found following
bug. On some arches, it is possible we publish a table while its content
is not yet committed to memory, and lockless reader can dereference wild
pointer.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_proto.c

@@ -292,6 +292,12 @@ int nf_conntrack_l4proto_register(struct nf_conntrack_l4proto *l4proto)
 
 		for (i = 0; i < MAX_NF_CT_PROTO; i++)
 			proto_array[i] = &nf_conntrack_l4proto_generic;
+
+		/* Before making proto_array visible to lockless readers,
+		 * we must make sure its content is committed to memory.
+		 */
+		smp_wmb();
+
 		nf_ct_protos[l4proto->l3proto] = proto_array;
 	} else if (nf_ct_protos[l4proto->l3proto][l4proto->l4proto] !=
 					&nf_conntrack_l4proto_generic) {