iomap: build the writeback code without CONFIG_BLOCK

Allow fuse to use the iomap writeback code even when CONFIG_BLOCK is
not enabled.  Do this with an ifdef instead of a separate file to keep
the iomap_folio_state local to buffered-io.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-15-hch@lst.de


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