block: use iomap_dirty_folio for block devices

With CONFIG_BUFFER_HEAD=n, block devices are written back through iomap,
but def_blk_aops uses filemap_dirty_folio, which only sets PG_dirty. It
does not set the per-block dirty bits in the folio's iomap_folio_state,
so iomap_writeback_folio() finds no dirty range, submits no I/O and
clears PG_dirty, resulting in data loss.

Other iomap users set .dirty_folio to iomap_dirty_folio, which marks the
folio's blocks dirty before calling filemap_dirty_folio().

This is only observable with block size < folio size. With a single
block there is no iomap_folio_state to get out of sync and
iomap_writeback_folio() marks the whole folio dirty itself. For a
page-aligned device, this may require using the BLKBSZSET ioctl to set
the block size, which requires CAP_SYS_ADMIN. A device whose size is not
page aligned already gets a sub-page block size from
set_init_blocksize(), so no ioctl and no privilege is needed.

To reproduce, on a device with a sub-page block size, write a known
pattern with O_DIRECT, mmap the same range, store to it, msync() and
fsync(), then read it back with O_DIRECT. A reproducer is available at
[1].

[1] https://gist.github.com/tzussman/18ab05cba4b3fdc79cce0a69d1fd05b4

Fixes: 925c86a19b ("fs: add CONFIG_BUFFER_HEAD")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260730-blk-dontcache-v7-0-3e8e6850068d%40columbia.edu?part=5
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Link: https://patch.msgid.link/20261009-blkdev-fixes-v6-1-307939c387df@columbia.edu
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -575,7 +575,7 @@ static int blkdev_writepages(struct address_space *mapping,
 }
 
 const struct address_space_operations def_blk_aops = {
-	.dirty_folio	= filemap_dirty_folio,
+	.dirty_folio		= iomap_dirty_folio,
 	.release_folio		= iomap_release_folio,
 	.invalidate_folio	= iomap_invalidate_folio,
 	.read_folio		= blkdev_read_folio,