selftests/landlock: Skip overlayfs tests when not supported

selftests/landlock: Skip overlayfs tests when not supported

overlayfs may be disabled in the kernel configuration, causing related
tests to fail.  Check that overlayfs is supported at runtime, so we can
skip layout2_overlay.* accordingly.

Signed-off-by: Jeff Xu <jeffxu@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230113053229.1281774-2-jeffxu@google.com
[mic: Reword comments and constify variables]
Signed-off-by: Mickaël Salaün <mic@digikod.net>