mm, swap: add support for stable large allocation in swap cache directly

To make it possible to allocate large folios directly in swap cache,
provide a new infrastructure helper to handle the swap cache status check,
allocation, and order fallback in the swap cache layer

The new helper replaces the existing swap_cache_alloc_folio.  Based on
this, all the separate swap folio allocation that is being done by anon /
shmem before is converted to use this helper directly, unifying folio
allocation for anon, shmem, and readahead.

This slightly consolidates how allocation is synchronized, making it more
stable and less prone to errors.  The slot-count and cache-conflict check
is now always performed with the cluster lock held before allocation, and
repeated under the same lock right before cache insertion.  This double
check produces a stable result compared to the previous anon and shmem
mTHP allocation implementation, avoids the false-negative conflict checks
that the lockless path can return — large allocations no longer have to
be unwound because the range turned out to be occupied — and aborts
early for already-freed slots, which helps ordinary swapin and especially
readahead, with only a marginal increase in cluster-lock contention (the
lock is very lightly contended and stays local in the first place). 
Hence, callers of swap_cache_alloc_folio() no longer need to check the
swap slot count or swap cache status themselves.

And now whoever first successfully allocates a folio in the swap cache
will be the one who charges it and performs the swap-in.  The race window
of swapping is also reduced since the loop is much more compact.

Link: https://lore.kernel.org/20260517-swap-table-p4-v5-4-88ae43e064c7@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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