* disk->disk_dev is initialized in alloc_disk(), device_add()'d in
	  add_disk(), device_del()'d in unregister_disk() and device_put() in
	  put_disk().
	* devices of partitions are made its children.
	* attributes of disk one: dev (dev_t of the thing), range (number of
	  minors) and size (in sectors).
	* attributes of partition ones: dev (ditto), start (in sectors) and
	  size (in sectors).
	* disk devices are put on a new bus - "block"
	* if caller of add_disk() had set disk->driverfs_dev, we set symlinks:
	  "device" from disk to underlying device and "block" from underlying
	  device to disk.
	* ->release() of disk_dev frees disk and disk->part.
	At that point we have sane driverfs subtree for each gendisk and
refcount of its root (disk->disk_dev) can act as gendisk refcount.