mm/mglru: simplify and improve dirty writeback handling

Right now the flusher wakeup mechanism for MGLRU is less responsive and
unlikely to trigger compared to classical LRU.  The classical LRU wakes
the flusher if one batch of folios passed to shrink_folio_list is
unevictable due to under writeback.  MGLRU instead check and handle this
after the whole reclaim loop is done.

We previously even saw OOM problems due to passive flusher, which were
fixed but still not perfect [1].

We have just unified the dirty folio counting and activation routine, now
just move the dirty flush into the loop right after shrink_folio_list. 
This improves the performance a lot for workloads involving heavy
writeback and prepares for throttling too.

Test with YCSB workloadb showed a major performance improvement:

Before this series:
Throughput(ops/sec): 62485.02962831822
AverageLatency(us): 500.9746963330107
pgpgin 159347462
workingset_refault_file 34522071

After this commit:
Throughput(ops/sec): 80857.08510208207
AverageLatency(us): 386.653262968934
pgpgin 112233121
workingset_refault_file 19516246

The performance is a lot better with significantly lower refault.  We also
observed similar or higher performance gain for other real-world
workloads.

We were concerned that the dirty flush could cause more wear for SSD: that
should not be the problem here, since the wakeup condition is when the
dirty folios have been pushed to the tail of LRU, which indicates that
memory pressure is so high that writeback is blocking the workload
already.

Link: https://lore.kernel.org/20260428-mglru-reclaim-v7-11-02fabb92dc43@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Link: https://lore.kernel.org/linux-mm/20241026115714.1437435-1-jingxiangzeng.cas@gmail.com/ [1]
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chen Ridong <chenridong@huaweicloud.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Stevens <stevensd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Leno Hou <lenohou@gmail.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yafang <laoar.shao@gmail.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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