block: extend functionality to map bvec iterator

Extend blk_rq_map_user_iov so that it can handle bvec iterator, using
the new blk_rq_map_user_bvec function. It maps the pages from bvec
iterator into a bio and place the bio into request.

This helper will be used by nvme for uring-passthrough path when IO is
done using pre-mapped buffers.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220930062749.152261-11-anuj20.g@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>