block: remove disk_part_iter

Just open code the xa_for_each in the remaining user.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406062303.811835-12-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>