blk-mq-dma: create blk_map_iter type

blk-mq-dma: create blk_map_iter type

The req_iterator happens to have a similar fields to what the dma
iterator needs, but we're not necessarily iterating a request's
bi_io_vec. Create a new type that can be amended for additional future
use.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20250813153153.3260897-2-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>