netfilter: nf_tables: variable sized set element keys / data

This patch changes sets to support variable sized set element keys / data
up to 64 bytes each by using variable sized set extensions. This allows
to use concatenations with bigger data items suchs as IPv6 addresses.

As a side effect, small keys/data now don't require the full 16 bytes
of struct nft_data anymore but just the space they need.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -388,6 +388,9 @@ enum nft_data_attributes {
 };
 #define NFTA_DATA_MAX		(__NFTA_DATA_MAX - 1)
 
+/* Maximum length of a value */
+#define NFT_DATA_VALUE_MAXLEN	64
+
 /**
  * enum nft_verdict_attributes - nf_tables verdict netlink attributes
  *