mm: vmscan: remove the redundant FOLIOREF_RECLAIM_CLEAN logic

folio_check_references() will return FOLIOREF_RECLAIM_CLEAN for referenced
file folios, indicating that we can proceed to reclaim clean file folios
or keep them if they are dirty file folios.  However, after commit
6b0dfabb3555 ("fs: Remove aops->writepage"), we no longer attempt to write
back filesystem folios through reclaim.  Instead, we always activate dirty
file folios and wakeup the flush workers to write them back.  As a result,
the FOLIOREF_RECLAIM_CLEAN logic is now redundant: for dirty file folios,
we will no longer reach the 'references == FOLIOREF_RECLAIM_CLEAN' branch
in shrink_folio_list().

Additionally, lazyfree folios are also placed on the file LRU list, but if
a lazyfree folio becomes dirty, try_to_unmap() will fail and thus prevent
reclaim of the re-dirtied lazyfree folios.

Therefore, we can drop the FOLIOREF_RECLAIM_CLEAN-related logic.

Link: https://lore.kernel.org/def70a713e10bcbdf3b9fccc2139ecc07b64f2cb.1782715791.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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