ubd curiously updates rq->sector while issuing the request in multiple
pieces.  Don't do it and simply use local copy of sector.

[ Impact: cleanup ]
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>