netfilter: nf_tables: bail out if stateful expression provides no .clone

All existing NFT_EXPR_STATEFUL provide a .clone interface, remove
fallback to copy content of stateful expression since this is never
exercised and bail out if .clone interface is not defined.

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

Author: ummakynes

net/netfilter/nf_tables_api.c

@@ -3274,14 +3274,13 @@ int nft_expr_clone(struct nft_expr *dst, struct nft_expr *src)
 {
 	int err;
 
-	if (src->ops->clone) {
-		dst->ops = src->ops;
-		err = src->ops->clone(dst, src);
-		if (err < 0)
-			return err;
-	} else {
-		memcpy(dst, src, src->ops->size);
-	}
+	if (WARN_ON_ONCE(!src->ops->clone))
+		return -EINVAL;
+
+	dst->ops = src->ops;
+	err = src->ops->clone(dst, src);
+	if (err < 0)
+		return err;
 
 	__module_get(src->ops->type->owner);