obj: add header-offset field to object_type

this allows the object header to be placed anywhere within the larger
object structure.

the object system now also ensures that the object is zero-initialised
during allocation.

obj/namespace.c

@@ -33,6 +33,7 @@ static kern_status_t ns_get_child_named(struct object *obj, const char *name, st
 static struct object_type ns_type = {
 	.ob_name = "namespace",
 	.ob_size = sizeof(struct object_namespace),
+	.ob_header_offset = offsetof(struct object_namespace, ns_base),
 	.ob_ops = {
 		.query_name = ns_query_name,
 		.get_named = ns_get_child_named,