virtio_blk: don't blindly derefence req->rq_disk

request->rq_disk is only set for FS requests or BLOCK_PC requests
originating from the generic block layer scsi ioctls.  It's not set
for requests origination from other soures or internal cache flush
commands implemented by the patch I'll send after this.

So instead of using it to get at the private data in do_virtblk_request
setup queue->queuedata and use it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/virtio_blk.c

@@ -124,12 +124,11 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 
 static void do_virtblk_request(struct request_queue *q)
 {
-	struct virtio_blk *vblk = NULL;
+	struct virtio_blk *vblk = q->queuedata;
 	struct request *req;
 	unsigned int issued = 0;
 
 	while ((req = blk_peek_request(q)) != NULL) {
-		vblk = req->rq_disk->private_data;
 		BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
 
 		/* If this request fails, stop queue and wait for something to
@@ -249,6 +248,7 @@ static int virtblk_probe(struct virtio_device *vdev)
 		goto out_put_disk;
 	}
 
+	vblk->disk->queue->queuedata = vblk;
 	queue_flag_set_unlocked(QUEUE_FLAG_VIRT, vblk->disk->queue);
 
 	if (index < 26) {