mm: rename swap.c to folio.c

Rename mm/swap.c to mm/folio.c so the filename better matches the code's
main responsibility.

This keeps the implementation split from swap-specific code without
changing the published LRU helper interfaces.

Update MAINTAINERS and the remaining mm/swap.c documentation references
after the rename.

[wujianyue000@gmail.com: MAINTAINERS: move mm/folio.c to MM CORE]
  Link: https://lore.kernel.org/20260804111109.393127-1-wujianyue000@gmail.com
Link: https://lore.kernel.org/20260708-ch-swap-series-plus-folio-lru-cleanup-v9-2-2bc72b4f8730@gmail.com
Signed-off-by: Jianyue Wu <wujianyue000@gmail.com>
Suggested-by: Baoquan He <bhe@redhat.com>
Suggested-by: David Hildenbrand <david@kernel.org>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@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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