selftests: add zswapin and no zswap tests

Add a selftest to cover the zswapin code path, allocating more memory than
the cgroup limit to trigger swapout/zswapout, then reading the pages back
in memory several times.  This is inspired by a recently encountered
kernel crash on the zswapin path in our internal kernel, which went
undetected because of a lack of test coverage for this path.

Add a selftest to verify that when memory.zswap.max = 0, no pages can go
to the zswap pool for the cgroup.

[[email protected]: remove redundant comment, add success checks]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Nhat Pham <[email protected]>
Suggested-by: Rik van Riel <[email protected]>
Suggested-by: Yosry Ahmed <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Zefan Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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