netfilter: nf_tables: add ARP filtering support

This patch registers the ARP family and he filter chain type
for this family.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/include/net/netns/nftables.h b/include/net/netns/nftables.h
index 08a4248..15d056d 100644
--- a/include/net/netns/nftables.h
+++ b/include/net/netns/nftables.h
@@ -10,6 +10,7 @@
 	struct list_head	commit_list;
 	struct nft_af_info	*ipv4;
 	struct nft_af_info	*ipv6;
+	struct nft_af_info	*arp;
 	struct nft_af_info	*bridge;
 	u8			gencursor;
 	u8			genctr;