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>

lib/overflow_kunit.c

@@ -6,6 +6,7 @@
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <kunit/device.h>
 #include <kunit/test.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
@@ -618,7 +619,7 @@ static void overflow_allocation_test(struct kunit *test)
 } while (0)
 
 	/* Create dummy device for devm_kmalloc()-family tests. */
-	dev = root_device_register(device_name);
+	dev = kunit_device_register(test, device_name);
 	KUNIT_ASSERT_FALSE_MSG(test, IS_ERR(dev),
 			       "Cannot register test device\n");
 
@@ -634,8 +635,6 @@ static void overflow_allocation_test(struct kunit *test)
 	check_allocation_overflow(devm_kmalloc);
 	check_allocation_overflow(devm_kzalloc);
 
-	device_unregister(dev);
-
 	kunit_info(test, "%d allocation overflow tests finished\n", count);
 #undef check_allocation_overflow
 }