nbd: don't clear rq->sector and nr_sectors unnecessarily

There's no reason to clear rq->sector and nr_sectors after calling
blk_rq_init().  They're guaranteed to be clear.  Drop unnecessary
clearing.

[ Impact: cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>