vfio/mdev: Do not allow a mdev_type to have a NULL parent pointer

There is a small race where the parent is NULL even though the kobj has
already been made visible in sysfs.

For instance the attribute_group is made visible in sysfs_create_files()
and the mdev_type_attr_show() does:

    ret = attr->show(kobj, type->parent->dev, buf);

Which will crash on NULL parent. Move the parent setup to before the type
pointer leaves the stack frame.

Fixes: 7b96953b ("vfio: Mediated device Core driver")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <2-v2-d36939638fc6+d54-vfio2_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/mdev/mdev_sysfs.c

@@ -105,6 +105,7 @@ static struct mdev_type *add_mdev_supported_type(struct mdev_parent *parent,
 		return ERR_PTR(-ENOMEM);
 
 	type->kobj.kset = parent->mdev_types_kset;
+	type->parent = parent;
 
 	ret = kobject_init_and_add(&type->kobj, &mdev_type_ktype, NULL,
 				   "%s-%s", dev_driver_string(parent->dev),
@@ -132,7 +133,6 @@ static struct mdev_type *add_mdev_supported_type(struct mdev_parent *parent,
 	}
 
 	type->group = group;
-	type->parent = parent;
 	return type;
 
 attrs_failed: