selftests: filesystems: fix warn_unused_result build warnings

Fix the following warnings by adding return check and error messages.

statmount_test.c: In function ‘cleanup_namespace’:
statmount_test.c:128:9: warning: ignoring return value of ‘fchdir’
declared with attribute ‘warn_unused_result’ [-Wunused-result]
  128 |         fchdir(orig_root);
      |         ^~~~~~~~~~~~~~~~~
statmount_test.c:129:9: warning: ignoring return value of ‘chroot’
declared with attribute ‘warn_unused_result’ [-Wunused-result]
  129 |         chroot(".");
      |         ^~~~~~~~~~~

Signed-off-by: Amer Al Shanawany <amer.shanawany@gmail.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: Amer-shanawany

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

@@ -125,8 +125,16 @@ static uint32_t old_root_id, old_parent_id;
 
 static void cleanup_namespace(void)
 {
-	fchdir(orig_root);
-	chroot(".");
+	int ret;
+
+	ret = fchdir(orig_root);
+	if (ret == -1)
+		ksft_perror("fchdir to original root");
+
+	ret = chroot(".");
+	if (ret == -1)
+		ksft_perror("chroot to original root");
+
 	umount2(root_mntpoint, MNT_DETACH);
 	rmdir(root_mntpoint);
 }