mm: memcg: add per-memcg zswap writeback stat

Since zswap now writes back pages from memcg-specific LRUs, we now need a
new stat to show writebacks count for each memcg.

[[email protected]: rename ZSWP_WB to ZSWPWB]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Nhat Pham <[email protected]>
Signed-off-by: Domenico Cerasuolo <[email protected]>
Signed-off-by: Nhat Pham <[email protected]>
Tested-by: Bagas Sanjaya <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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