iomap: lift error code check out of iomap_iter_advance()

The error code is only used to check whether iomap_iter() should
terminate due to an error returned in iter.processed. Lift the check
out of iomap_iter_advance() in preparation to make it more generic.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250207143253.314068-5-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>