[PATCH v6 0/6] Implement writeback for zsmalloc (fix)

[Nhat Pham <nphamcs@gmail.com>]

This contains two small fixes that should be applied on top of patch 4
and patch 6 of my zsmalloc writeback series, respectively.

Changelog:
v6-fix:
  * Add a comment explaining LRU update logic in zs_map_object.
    (patch 4)
    (sugegested by Sergey Senozhatsky and Johannes Weiner).
  * Use get_first_page() and add cond_resched() in retry-loop.
    (patch 6) (sugegested by Sergey Senozhatsky).
v6:
  * Move the move-to-front logic into zs_map_object (patch 4)
    (suggested by Minchan Kim).
  * Small clean up for free_zspage at free_handles() call site
    (patch 6) (suggested by Minchan Kim).
v5:
  * Add a new patch that eliminates unused code in zpool and simplify
    the logic for storing evict handler in zbud/z3fold (patch 2)
  * Remove redudant fields in zs_pool (previously required by zpool)
    (patch 3)
  * Wrap under_reclaim and deferred handle freeing logic in CONFIG_ZPOOL
    (patch 6) (suggested by Minchan Kim)
  * Move a small piece of refactoring from patch 6 to patch 4.
v4:
  * Wrap the new LRU logic in CONFIG_ZPOOL (patch 3).
    (suggested by Minchan Kim)
v3:
  * Set pool->ops = NULL when pool->zpool_ops is null (patch 4).
  * Stop holding pool's lock when calling lock_zspage() (patch 5).
    (suggested by Sergey Senozhatsky)
  * Stop holding pool's lock when checking pool->ops and retries.
    (patch 5) (suggested by Sergey Senozhatsky)
  * Fix formatting issues (.shrink, extra spaces in casting removed).
    (patch 5) (suggested by Sergey Senozhatsky)
v2:
  * Add missing CONFIG_ZPOOL ifdefs (patch 5)
    (detected by kernel test robot).

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2.30.2