netfilter: nf_tables: allow to specify stateful expression in set definition

This patch allows users to specify the stateful expression for the
elements in this set via NFTA_SET_EXPR. This new feature allows you to
turn on counters for all of the elements in this set.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -416,6 +416,7 @@ struct nft_set_type {
  *	@policy: set parameterization (see enum nft_set_policies)
  *	@udlen: user data length
  *	@udata: user data
+ *	@expr: stateful expression
  * 	@ops: set ops
  * 	@flags: set flags
  *	@genmask: generation mask
@@ -444,6 +445,7 @@ struct nft_set {
 	u16				policy;
 	u16				udlen;
 	unsigned char			*udata;
+	struct nft_expr			*expr;
 	/* runtime data below here */
 	const struct nft_set_ops	*ops ____cacheline_aligned;
 	u16				flags:14,