netfilter: add protocol independent NAT core

Convert the IPv4 NAT implementation to a protocol independent core and
address family specific modules.

Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/net/netns/conntrack.h b/include/net/netns/conntrack.h
index 3aecdc7..a1d83cc 100644
--- a/include/net/netns/conntrack.h
+++ b/include/net/netns/conntrack.h
@@ -83,6 +83,10 @@
 	int			sysctl_auto_assign_helper;
 	bool			auto_assign_helper_warned;
 	struct nf_ip_net	nf_ct_proto;
+#ifdef CONFIG_NF_NAT_NEEDED
+	struct hlist_head	*nat_bysource;
+	unsigned int		nat_htable_size;
+#endif
 #ifdef CONFIG_SYSCTL
 	struct ctl_table_header	*sysctl_header;
 	struct ctl_table_header	*acct_sysctl_header;