generic/755: add _require_hardlinks

This test is used to test whether the ctime of the original file
changes after deleting its hard link, which requires hardlink
support, so add _require_hardlinks to check for hardlink support.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Friendy Su <friendy.su@sony.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>