[NETFILTER]: nf_conntrack_extend: use __read_mostly for struct nf_ct_ext_type

Also make them static.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c
index 6d32399..dc352f5 100644
--- a/net/netfilter/nf_conntrack_helper.c
+++ b/net/netfilter/nf_conntrack_helper.c
@@ -145,7 +145,7 @@
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_helper_unregister);
 
-struct nf_ct_ext_type helper_extend = {
+static struct nf_ct_ext_type helper_extend __read_mostly = {
 	.len	= sizeof(struct nf_conn_help),
 	.align	= __alignof__(struct nf_conn_help),
 	.id	= NF_CT_EXT_HELPER,