selftests: cgroup: add tests to verify the zswap writeback path

Attempt writeback with the below steps and check using memory.stat.zswpwb
if zswap writeback occurred:

1. Allocate memory.
2. Reclaim memory equal to the amount that was allocated in step 1.
   This will move it into zswap.
3. Save current zswap usage.
4. Move the memory allocated in step 1 back in from zswap.
5. Set zswap.max to half the amount that was recorded in step 3.
6. Attempt to reclaim memory equal to the amount that was allocated,
   this will either trigger writeback if it's enabled, or reclamation
   will fail if writeback is disabled as there isn't enough zswap
   space.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Usama Arif <[email protected]>
Suggested-by: Nhat Pham <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Acked-by: Nhat Pham <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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