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>