scsi: stop setting up request->special

No more need in a blk-mq world where the scsi command and request are
allocated together.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>