netfilter: nft_meta: add cgroup support

This allows you to filter traffic by process control group (cgroup).

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/include/uapi/linux/netfilter/nf_tables.h b/include/uapi/linux/netfilter/nf_tables.h
index 16f62a5..832bc46 100644
--- a/include/uapi/linux/netfilter/nf_tables.h
+++ b/include/uapi/linux/netfilter/nf_tables.h
@@ -579,6 +579,7 @@
  * @NFT_META_CPU: cpu id through smp_processor_id()
  * @NFT_META_IIFGROUP: packet input interface group
  * @NFT_META_OIFGROUP: packet output interface group
+ * @NFT_META_CGROUP: socket control group (skb->sk->sk_classid)
  */
 enum nft_meta_keys {
 	NFT_META_LEN,
@@ -604,6 +605,7 @@
 	NFT_META_CPU,
 	NFT_META_IIFGROUP,
 	NFT_META_OIFGROUP,
+	NFT_META_CGROUP,
 };
 
 /**