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/net/netfilter/core.c b/net/netfilter/core.c
index 8f4b0b2..e61b3ac 100644
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -275,6 +275,11 @@
 
 #endif /* CONFIG_NF_CONNTRACK */
 
+#ifdef CONFIG_NF_NAT_NEEDED
+void (*nf_nat_decode_session_hook)(struct sk_buff *, struct flowi *);
+EXPORT_SYMBOL(nf_nat_decode_session_hook);
+#endif
+
 #ifdef CONFIG_PROC_FS
 struct proc_dir_entry *proc_net_netfilter;
 EXPORT_SYMBOL(proc_net_netfilter);