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/[email protected]
Signed-off-by: Mike Yuan <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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