drbd: poison free'd device, resource and connection structs

Now that we have additional asynchronous kref_get/kref_put
via debugfs, make sure we catch access after free.

Poison struct drbd_device, drbd_connection and drbd_resource
before kfree() with 0xfd, 0xfc, and 0xf2, respectively.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -2192,6 +2192,7 @@ void drbd_destroy_device(struct kref *kref)
 	blk_cleanup_queue(device->rq_queue);
 	kfree(device->rs_plan_s);
 	kfree(first_peer_device(device));
+	memset(device, 0xfd, sizeof(*device));
 	kfree(device);
 
 	for_each_connection(connection, resource)
@@ -2285,6 +2286,7 @@ void drbd_destroy_resource(struct kref *kref)
 	idr_destroy(&resource->devices);
 	free_cpumask_var(resource->cpu_mask);
 	kfree(resource->name);
+	memset(resource, 0xf2, sizeof(*resource));
 	kfree(resource);
 }
 
@@ -2665,6 +2667,7 @@ void drbd_destroy_connection(struct kref *kref)
 	drbd_free_socket(&connection->data);
 	kfree(connection->int_dig_in);
 	kfree(connection->int_dig_vv);
+	memset(connection, 0xfc, sizeof(*connection));
 	kfree(connection);
 	kref_put(&resource->kref, drbd_destroy_resource);
 }