selftests: always check mask returned by statmount(2)

STATMOUNT_MNT_OPTS can actually be missing if there are no options. This
is a change of behavior since 75ead69a7173 ("fs: don't let statmount return
empty strings").

The other checks shouldn't actually trigger, but add them for correctness
and for easier debugging if the test fails.

Signed-off-by: Miklos Szeredi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/filesystems/statmount/statmount_test.c

···
 		return;
 	}
 
+	if (!(sm->mask & STATMOUNT_MNT_POINT)) {
+		ksft_test_result_fail("missing STATMOUNT_MNT_POINT in mask\n");
+		return;
+	}
 	if (strcmp(sm->str + sm->mnt_point, "/") != 0) {
 		ksft_test_result_fail("unexpected mount point: '%s' != '/'\n",
 				      sm->str + sm->mnt_point);
···
 	if (!sm) {
 		ksft_test_result_fail("statmount mount root: %s\n",
 				      strerror(errno));
+		return;
+	}
+	if (!(sm->mask & STATMOUNT_MNT_ROOT)) {
+		ksft_test_result_fail("missing STATMOUNT_MNT_ROOT in mask\n");
 		return;
 	}
 	mnt_root = sm->str + sm->mnt_root;
···
 				      strerror(errno));
 		return;
 	}
+	if (!(sm->mask & STATMOUNT_FS_TYPE)) {
+		ksft_test_result_fail("missing STATMOUNT_FS_TYPE in mask\n");
+		return;
+	}
 	fs_type = sm->str + sm->fs_type;
 	for (s = known_fs; s != NULL; s++) {
 		if (strcmp(fs_type, *s) == 0)
···
 	if (!sm) {
 		ksft_test_result_fail("statmount mnt opts: %s\n",
 				      strerror(errno));
+		return;
+	}
+
+	if (!(sm->mask & STATMOUNT_MNT_BASIC)) {
+		ksft_test_result_fail("missing STATMOUNT_MNT_BASIC in mask\n");
 		return;
 	}
 
···
 		if (p2)
 			*p2 = '\0';
 
-		statmount_opts = sm->str + sm->mnt_opts;
+		if (sm->mask & STATMOUNT_MNT_OPTS)
+			statmount_opts = sm->str + sm->mnt_opts;
+		else
+			statmount_opts = "";
 		if (strcmp(statmount_opts, p) != 0)
 			ksft_test_result_fail(
 				"unexpected mount options: '%s' != '%s'\n",