netfilter: nf_tables: reintroduce the NFT_SET_CONCAT flag

Stefano originally proposed to introduce this flag, users hit EOPNOTSUPP
in new binaries with old kernels when defining a set with ranges in
a concatenation.

Fixes: f3a2181e16 ("netfilter: nf_tables: Support for sets with multiple ranged fields")
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -276,6 +276,7 @@ enum nft_rule_compat_attributes {
  * @NFT_SET_TIMEOUT: set uses timeouts
  * @NFT_SET_EVAL: set can be updated from the evaluation path
  * @NFT_SET_OBJECT: set contains stateful objects
+ * @NFT_SET_CONCAT: set contains a concatenation
  */
 enum nft_set_flags {
 	NFT_SET_ANONYMOUS		= 0x1,
@@ -285,6 +286,7 @@ enum nft_set_flags {
 	NFT_SET_TIMEOUT			= 0x10,
 	NFT_SET_EVAL			= 0x20,
 	NFT_SET_OBJECT			= 0x40,
+	NFT_SET_CONCAT			= 0x80,
 };
 
 /**