rbd: resurrect setting of disk->private_data in rbd_init_disk()

rbd_open() and rbd_release() expect that disk->private_data is set to
rbd_dev.  Otherwise we hit a NULL pointer dereference when mapping the
image.

URL: https://tracker.ceph.com/issues/51759
Fixes: 195b1956 ("rbd: use blk_mq_alloc_disk and blk_cleanup_disk")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

drivers/block/rbd.c

@@ -4943,6 +4943,7 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
 		disk->minors = RBD_MINORS_PER_MAJOR;
 	}
 	disk->fops = &rbd_bd_ops;
+	disk->private_data = rbd_dev;
 
 	blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
 	/* QUEUE_FLAG_ADD_RANDOM is off by default for blk-mq */