dax: advance the iomap_iter on zero range

dax: advance the iomap_iter on zero range

Update the DAX zero range iomap iter handler to advance the iter
directly. Advance by the full length in the hole/unwritten case, or
otherwise advance incrementally in the zeroing loop. In either case,
return 0 or an error code for success or failure.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250224144757.237706-7-bfoster@redhat.com
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>