block: add blk_rq_map_user_io

Create a helper blk_rq_map_user_io for mapping of vectored as well as
non-vectored requests. This will help in saving dupilcation of code at few
places in scsi and nvme.

Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220930062749.152261-4-anuj20.g@samsung.com


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