iomap: convert iomap_writepages to writeack_iter

This removes one indirect function call per folio, and adds type safety
by not casting through a void pointer.

Based on a patch by Matthew Wilcox.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240412061614.1511629-1-hch@lst.de
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>