netfilter: nf_tables: add nft_set_elem_update_expr() helper function

This helper function runs the eval path of the stateful expression
of an existing set element.

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

include/net/netfilter/nf_tables.h

@@ -890,6 +890,18 @@ static inline struct nft_userdata *nft_userdata(const struct nft_rule *rule)
 	return (void *)&rule->data[rule->dlen];
 }
 
+static inline void nft_set_elem_update_expr(const struct nft_set_ext *ext,
+					    struct nft_regs *regs,
+					    const struct nft_pktinfo *pkt)
+{
+	struct nft_expr *expr;
+
+	if (nft_set_ext_exists(ext, NFT_SET_EXT_EXPR)) {
+		expr = nft_set_ext_expr(ext);
+		expr->ops->eval(expr, regs, pkt);
+	}
+}
+
 /*
  * The last pointer isn't really necessary, but the compiler isn't able to
  * determine that the result of nft_expr_last() is always the same since it