netfilter: nft_ct: add zone id get support

Just like with counters the direction attribute is optional.
We set priv->dir to MAX unconditionally to avoid duplicating the assignment
for all keys with optional direction.

For keys where direction is mandatory, existing code already returns
an error.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -870,6 +870,7 @@ enum nft_rt_attributes {
  * @NFT_CT_PKTS: conntrack packets
  * @NFT_CT_BYTES: conntrack bytes
  * @NFT_CT_AVGPKT: conntrack average bytes per packet
+ * @NFT_CT_ZONE: conntrack zone
  */
 enum nft_ct_keys {
 	NFT_CT_STATE,
@@ -889,6 +890,7 @@ enum nft_ct_keys {
 	NFT_CT_PKTS,
 	NFT_CT_BYTES,
 	NFT_CT_AVGPKT,
+	NFT_CT_ZONE,
 };
 
 /**