fuse: use iomap for folio laundering

Use iomap for folio laundering, which will do granular dirty
writeback when laundering a large folio.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/20250715202122.2282532-4-joannelkoong@gmail.com


Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>