mm, swap: fix swap cache index error when retrying reclaim

The allocator will reclaim cached slots while scanning.  Currently, it
will try again if reclaim found a folio that is already removed from the
swap cache due to a race.  But the following lookup will be using the
wrong index.  It won't cause any OOB issue since the swap cache index is
truncated upon lookup, but it may lead to reclaiming of an irrelevant
folio.

This should not cause a measurable issue, but we should fix it.

Link: https://lkml.kernel.org/r/20250916160100.31545-4-ryncsn@gmail.com
Fixes: fae859550531 ("mm, swap: avoid reclaiming irrelevant swap cache")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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