iomap: factor out a iomap_dio_done helper

Split out the struct iomap-dio level final completion from
iomap_dio_bio_end_io into a helper to clean up the code and make it
reusable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250206064035.2323428-7-hch@lst.de


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