fuse: use iomap for writeback

fuse: use iomap for writeback

Use iomap for dirty folio writeback in ->writepages().
This allows for granular dirty writeback of large folios.

Only the dirty portions of the large folio will be written instead of
having to write out the entire folio. For example if there is a 1 MB
large folio and only 2 bytes in it are dirty, only the page for those
dirty bytes will be written out.

.dirty_folio needs to be set to iomap_dirty_folio so that the bitmap
iomap uses for dirty tracking correctly reflects dirty regions that need
to be written back.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/20250715202122.2282532-3-joannelkoong@gmail.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>