selftests: test_zswap: add test for hierarchical zswap.writeback

Ensure that zswap.writeback check goes up the cgroup tree, i.e.  is
hierarchical.  Create a subcgroup which has zswap.writeback set to 1, and
the upper hierarchy's restrictions shall apply.

Link: https://lkml.kernel.org/r/20240823162506.12117-2-me@yhndnzj.com


Signed-off-by: Mike Yuan <me@yhndnzj.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>