vfio/mdev: Add missing reference counting to mdev_type

struct mdev_type holds a pointer to the kref'd object struct mdev_parent,
but doesn't hold the kref. The lifetime of the parent becomes implicit
because parent_remove_sysfs_files() is supposed to remove all the access
before the parent can be freed, but this is very hard to reason about.

Make it obviously correct by adding the missing get.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <7-v2-d36939638fc6+d54-vfio2_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/mdev/mdev_sysfs.c

@@ -81,6 +81,8 @@ static void mdev_type_release(struct kobject *kobj)
 	struct mdev_type *type = to_mdev_type(kobj);
 
 	pr_debug("Releasing group %s\n", kobj->name);
+	/* Pairs with the get in add_mdev_supported_type() */
+	mdev_put_parent(type->parent);
 	kfree(type);
 }
 
@@ -106,6 +108,8 @@ static struct mdev_type *add_mdev_supported_type(struct mdev_parent *parent,
 
 	type->kobj.kset = parent->mdev_types_kset;
 	type->parent = parent;
+	/* Pairs with the put in mdev_type_release() */
+	mdev_get_parent(parent);
 
 	ret = kobject_init_and_add(&type->kobj, &mdev_type_ktype, NULL,
 				   "%s-%s", dev_driver_string(parent->dev),