f_fw: fix compat mode

The kernel takes a lack of options as indication that the fw classifier
should operate in compatibility mode, where marks are mapped directly to
classids.

Commit e22b42a2 (tc mask patch) broke this by adding an empty TCA_OPTIONS
attribute even if no handle is specified. Restore the old behaviour.
Signed-off-by: Patrick McHardy <kaber@trash.net>

tc/f_fw.c

@@ -38,15 +38,13 @@ static int fw_parse_opt(struct filter_util *qu, char *handle, int argc, char **a
 	struct tc_police tp;
 	struct tcmsg *t = NLMSG_DATA(n);
 	struct rtattr *tail;
+	__u32 mask = 0;
+	int mask_set = 0;
 
 	memset(&tp, 0, sizeof(tp));
 
-	tail = NLMSG_TAIL(n);
-	addattr_l(n, 4096, TCA_OPTIONS, NULL, 0);
-
 	if (handle) {
 		char *slash;
-		__u32 mask = 0;
 		if ((slash = strchr(handle, '/')) != NULL)
 			*slash = '\0';
 		if (get_u32(&t->tcm_handle, handle, 0)) {
@@ -58,13 +56,19 @@ static int fw_parse_opt(struct filter_util *qu, char *handle, int argc, char **a
 				fprintf(stderr, "Illegal \"handle\" mask\n");
 				return -1;
 			}
-			addattr32(n, MAX_MSG, TCA_FW_MASK, mask);
+			mask_set = 1;
 		}
 	}
 
 	if (argc == 0)
 		return 0;
 
+	tail = NLMSG_TAIL(n);
+	addattr_l(n, 4096, TCA_OPTIONS, NULL, 0);
+
+	if (mask_set)
+		addattr32(n, MAX_MSG, TCA_FW_MASK, mask);
+
 	while (argc > 0) {
 		if (matches(*argv, "classid") == 0 ||
 		    matches(*argv, "flowid") == 0) {