overflow: Replace fake root_device with kunit_device

Using struct root_device to create fake devices for tests is something
of a hack. The new struct kunit_device is meant for this purpose, so use
it instead.

Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>