mm, swap: consolidate cluster reclaim and usability check

Swap cluster cache reclaim requires releasing the lock, so the cluster may
become unusable after the reclaim.  To prepare for checking swap cache
using the swap table directly, consolidate the swap cluster reclaim and
the check logic.

We will want to avoid touching the cluster's data completely with the swap
table, to avoid RCU overhead here.  And by moving the cluster usable check
into the reclaim helper, it will also help avoid a redundant scan of the
slots if the cluster is no longer usable, and we will want to avoid
touching the cluster.

Also, adjust it very slightly while at it: always scan the whole region
during reclaim, don't skip slots covered by a reclaimed folio.  Because
the reclaim is lockless, it's possible that new cache lands at any time. 
And for allocation, we want all caches to be reclaimed to avoid
fragmentation.  Besides, if the scan offset is not aligned with the size
of the reclaimed folio, we might skip some existing cache and fail the
reclaim unexpectedly.

There should be no observable behavior change.  It might slightly improve
the fragmentation issue or performance.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-10-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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