mm: split out a writeout helper from pageout

mm: split out a writeout helper from pageout

Patch series "stop passing a writeback_control to swap/shmem writeout",
v3.

This series was intended to remove the last remaining users of
AOP_WRITEPAGE_ACTIVATE after my other pending patches removed the rest,
but spectacularly failed at that.

But instead it nicely improves the code, and removes two pointers from
struct writeback_control.

This patch (of 6):

Move the code to write back swap / shmem folios into a self-contained
helper to keep prepare for refactoring it.

Link: https://lkml.kernel.org/r/20250610054959.2057526-1-hch@lst.de
Link: https://lkml.kernel.org/r/20250610054959.2057526-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Baolin Wang <baolin.wang@linux.aibaba.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>