blk-integrity: use iterator for mapping sg

blk-integrity: use iterator for mapping sg

Modify blk_rq_map_integrity_sg to use the blk-mq mapping iterator. This
produces more efficient code and converges the integrity mapping
implementations to reduce future maintenance burdens.

The function implementation moves from blk-integrity.c to blk-mq-dma.c
in order to use the types and functions private to that file.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250813153153.3260897-8-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>