iomap: export iomap_iter_advance() and return remaining length

As a final step for generic iter advance, export the helper and
update it to return the remaining length of the current iteration
after the advance. This will usually be 0 in the iomap_iter() case,
but will be useful for the various operations that iterate on their
own and will be updated to advance as they progress.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250207143253.314068-7-bfoster@redhat.com


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