* added ->media_changed(disk).  Callers of ->check_media_changed()
are using that one if present
	* added ->revaldiate_disk(disk).  Callers of ->revalidate()
are using that one if present
	* added disk->queue.  do_open() uses it if non-NULL
	* added disk->private_data
	* removed assignments to inode->i_data.backing_... - it belongs to
filesystem that contains device node and we don't care about it;
inode->i_mapping points to ->i_data of bdev inode and there we should
(and do) set that field.