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>