iomap: export iomap_writeback_folio

Allow fuse to use iomap_writeback_folio for folio laundering.  Note
that the caller needs to manually submit the pending writeback context.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-11-hch@lst.de


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