New object: struct cdev.  It contains a kobject, a pointer to
file_operations and a pointer to owner module.  These guys have a search
structure of the same sort as gendisks and chrdev_open() picks
file_operations from them.

Intended use: embed such animal in driver-owned structure (e.g.
tty_driver) and register it as associated with given range of device
numbers.  Generic code will do lookup for such object and use it for the
rest.

The behaviour of register_chrdev() is _not_ changed - it allocates
struct cdev and registers it; any old driver will work as if nothing had
changed.

On that stage we only use it during chrdev_open() to find
file_operations.  Later it will be cached in inode->i_cdev (and index in
range - in inode->i_cindex) so that ->open() could get whatever objects
it wants directly without any special-cased lookups, etc.