block: use iter_ubuf for single range

This is more efficient than iter_iov.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[axboe: fold in iovec assumption fix]
Signed-off-by: Jens Axboe <axboe@kernel.dk>