[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/ipv4/netfilter/nf_nat_core.c b/net/ipv4/netfilter/nf_nat_core.c
index 7e31777..04691ed 100644
--- a/net/ipv4/netfilter/nf_nat_core.c
+++ b/net/ipv4/netfilter/nf_nat_core.c
@@ -623,7 +623,7 @@
 	write_unlock_bh(&nf_nat_lock);
 }
 
-struct nf_ct_ext_type nat_extend = {
+static struct nf_ct_ext_type nat_extend __read_mostly = {
 	.len		= sizeof(struct nf_conn_nat),
 	.align		= __alignof__(struct nf_conn_nat),
 	.destroy	= nf_nat_cleanup_conntrack,