mm: swap: drop dropbehind swap cache folios on writeback completion

A PG_dropbehind folio is dropped from its cache once writeback completes
rather than left for reclaim to find later; this is implemented for file
folios in folio_end_dropbehind(). Extend it to swap cache folios.

The drop blocks on the folio lock, so it cannot run in interrupt
context. Set BIO_COMPLETE_IN_TASK on the write, as the file dropbehind
paths do, and drop the folio directly from folio_end_writeback().

It has to block rather than trylock: the folio is off the LRU, so
skipping it would leave it in the swap cache with nothing able to
reclaim it, and it cannot be put back while another thread holds its
lock.

Link: https://lore.kernel.org/20260921151306.625134-3-alex@ghiti.fr
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Yosry Ahmed <yosry@kernel.org>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chis Li <chrisl@kernel.org>
Cc: Christian Brauner (Amutable) <brauner@kernel.org>
Cc: "David Hildenbrand (arm)" <david@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tal Zussman <tz2294@columbia.edu>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: Yuanchu Xie <yuanchu@google.com>

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