netfilter: nft_ct: Add support to set the connmark

This patch adds kernel support for setting properties of tracked
connections. Currently, only connmark is supported. One use-case
for this feature is to provide the same functionality as
-j CONNMARK --save-mark in iptables.

Some restructuring was needed to implement the set op. The new
structure follows that of nft_meta.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -609,12 +609,14 @@ enum nft_ct_keys {
  * @NFTA_CT_DREG: destination register (NLA_U32)
  * @NFTA_CT_KEY: conntrack data item to load (NLA_U32: nft_ct_keys)
  * @NFTA_CT_DIRECTION: direction in case of directional keys (NLA_U8)
+ * @NFTA_CT_SREG: source register (NLA_U32)
  */
 enum nft_ct_attributes {
 	NFTA_CT_UNSPEC,
 	NFTA_CT_DREG,
 	NFTA_CT_KEY,
 	NFTA_CT_DIRECTION,
+	NFTA_CT_SREG,
 	__NFTA_CT_MAX
 };
 #define NFTA_CT_MAX		(__NFTA_CT_MAX - 1)