This a multi-pronged attack aimed at exploiting the kobject infrastructure mor. 

- Remove bus_driver_list, in favor of list in bus_subys.

- Remove bus_for_each_* and driver_for_each_dev(). They're not being used by
anyone, have questionable locking semantics, and really don't provide that
much use, as the function returns once the callback fails, with no indication
of where it failed. Forget them, at least for now. 

- Make sure that we return success from bus_match() if device matches, but
doesn't have a probe method.

- Remove extraneous get_{device,driver}s from bus routines that are serialized
by the bus's rwsem. bus_{add,remove}_{device,driver} all take the rwsem, so there 
is no way we can get a non-existant object when in those functions.

- Use the rwsem in the struct subsystem the bus has embedded in it, and kill the
separate one in struct bus_type.

- Move bulk of driver_register() into bus_add_driver(), which holds the bus's
rwsem during the entirety. this will prevent the driver from being unloaded while
it's being registered, and two drivers with the same name getting registered
at the same time. 

- Ditto for driver_unregister() and bus_remove_driver().

- Add driver_release() method for the driver bus driver subsystems. (Explained later)

- Use only the refcounts in the buses' kobjects, and kill the one in struct bus_type.

- Kill struct bus_type::present and struct device_driver::present. These didn't 
work out the way we intended them to. The idea was to not let a user obtain a 
rerference count to the object if it was in the process of being unregistered. 
All the code paths should be fixed now such that their registration is protected with
a semaphore, so no partially initialized objects can be removed, and enough 
infrastructure is moved to the kobject model so that once the object is publically
visible, it should be usable by other sources.

- Add a bus_sem to serialize bus registration and unregistration. 

- Add struct device_driver::unload_sem to prevent unloading of drivers 
with a positive reference count.

The driver model has always had a bug that would allow a driver with a 
positive reference count to be unloaded. It would decrement the reference 
count and return, letting the module be unloaded, without accounting for 
the other users of the object. This has been discussed many times, though 
never resolved cleanly. This should fix the problem in the simplest manner.

struct device_driver gets unload_sem, which is initialized to _locked_. When
the reference count for the driver reaches 0, the semaphore is unlocked.
driver_unregister() blocks on acquiring this lock before it exits. In the 
normal case that driver_unregister() drops the last reference to the driver,
the lock will be acquired immediately, and the module will unload. 

In the case that someone else is using the driver object, driver_unregister()
will not be able to acquire the lock, since the refcount has not reached 0,
and the lock has not been released. 

This means that rmmod(8) will block while drivers' sysfs files are open. 
There are no sysfs files for drivers yet, but note this when they do have 
some.