scsi: Use blk_rq_map_user_io helper

Use the new blk_rq_map_user_io helper instead of duplicating code at
various places. Additionally this also takes advantage of the on-stack
iov fast path.

Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220930062749.152261-5-anuj20.g@samsung.com


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