mm/swap: also use struct swap_iocb for block I/O

Block I/O benefits from batching just as much as remote file systems. 
Extend struct swap_iocb to support building a bio on the fly as well, and
rewrite the block based swap code for it.  This especially benefits
submit_bio based drivers that do not have the block plugging available,
but also saves allocating extra bios for blk-mq drivers.

Add a pre-allocated bio to struct swap_iocb in a union with kiocb used for
file system based swap so that struct swap_iocb can be used for all swap
I/O, and initialize the pool for it unconditionally.

Various low-level bdev and fs functions are now replaced with a unified
can_merge/add/submit scheme.

Note that the block based swap code now uses the same memcg-based check
previously added for file system based swap as well.

Link: https://lore.kernel.org/20260713093350.2154226-4-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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