obj: object header is no longer allocated automatically

sched/thread.c

@@ -3,6 +3,8 @@
 #include <socks/cpu.h>
 #include <socks/machine/thread.h>
 
+#define THREAD_CAST(p) OBJECT_C_CAST(struct thread, thr_base, &thread_type, p)
+
 static struct object_type thread_type = {
 	.ob_name = "thread",
 	.ob_size = sizeof(struct thread),
@@ -20,7 +22,7 @@ struct thread *thread_alloc(void)
 		return NULL;
 	}
 
-	struct thread *t = object_data(thread_obj);
+	struct thread *t = THREAD_CAST(thread_obj);
 	memset(t, 0x00, sizeof *t);
 	return t;
 }