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.

kxld/kext.c

@@ -34,6 +34,7 @@ static kern_status_t kext_destroy(struct object *obj)
 static struct object_type kext_type = {
 	.ob_name = "kext",
 	.ob_size = sizeof(struct kext),
+	.ob_header_offset = offsetof(struct kext, k_base),
 	.ob_ops = {
 		.query_name = kext_query_name,
 	},