ubd: remove use of blk_rq_map_sg

There is no good reason to create a scatterlist in the ubd driver,
it can just iterate the request directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[rw: Folded in improvements as discussed with hch and jens]
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jens Axboe <axboe@kernel.dk>