block: require blk_rq_prep_clone() be given an initialized clone request

Prepare to allow blk_rq_prep_clone() to accept clone requests that were
allocated from blk-mq request queues.  As such the blk_rq_prep_clone()
caller must first initialize the clone request.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>