iommufd/selftest: Don't leak the platform device memory when unloading the module

It should call platform_device_unregister() instead of
platform_device_del() to unregister and free the device.

Fixes: 23a1b46f ("iommufd/selftest: Make the mock iommu driver into a real driver")
Link: https://lore.kernel.org/r/20230816081318.1232865-1-yangyingliang@huawei.comSigned-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/selftest.c

@@ -1089,7 +1089,7 @@ int __init iommufd_test_init(void)
 err_bus:
 	bus_unregister(&iommufd_mock_bus_type.bus);
 err_platform:
-	platform_device_del(selftest_iommu_dev);
+	platform_device_unregister(selftest_iommu_dev);
 err_dbgfs:
 	debugfs_remove_recursive(dbgfs_root);
 	return rc;
@@ -1102,6 +1102,6 @@ void iommufd_test_exit(void)
 				    &iommufd_mock_bus_type.bus,
 				    &iommufd_mock_bus_type.nb);
 	bus_unregister(&iommufd_mock_bus_type.bus);
-	platform_device_del(selftest_iommu_dev);
+	platform_device_unregister(selftest_iommu_dev);
 	debugfs_remove_recursive(dbgfs_root);
 }