kunit: device: Unregister the kunit_bus on shutdown

If KUnit is built as a module, and it's unloaded, the kunit_bus is not
unregistered. This causes an error if it's then re-loaded later, as we
try to re-register the bus.

Unregister the bus and root_device on shutdown, if it looks valid.

In addition, be more specific about the value of kunit_bus_device. It
is:
- a valid struct device* if the kunit_bus initialised correctly.
- an ERR_PTR if it failed to initialise.
- NULL before initialisation and after shutdown.

Fixes: d03c720e03bd ("kunit: Add APIs for managing devices")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: Add APIs for managing devices

Tests for drivers often require a struct device to pass to other
functions. While it's possible to create these with
root_device_register(), or to use something like a platform device, this
is both a misuse of those APIs, and can be difficult to clean up after,
for example, a failed assertion.

Add some KUnit-specific functions for registering and unregistering a
struct device:
- kunit_device_register()
- kunit_device_register_with_driver()
- kunit_device_unregister()

These helpers allocate a on a 'kunit' bus which will either probe the
driver passed in (kunit_device_register_with_driver), or will create a
stub driver (kunit_device_register) which is cleaned up on test shutdown.

Devices are automatically unregistered on test shutdown, but can be
manually unregistered earlier with kunit_device_unregister() in order
to, for example, test device release code.

Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>