Revert "scheduler: task.Data made 64bit to avoid overflow"

This reverts commit fd8938c634.

src/internal/task/task.go

@@ -13,7 +13,7 @@ type Task struct {
 	Ptr unsafe.Pointer
 
 	// Data is a field which can be used for storing state information.
-	Data uint64
+	Data uint
 
 	// state is the underlying running state of the task.
 	state state

src/runtime/scheduler.go

@@ -88,7 +88,7 @@ func addSleepTask(t *task.Task, duration timeUnit) {
 			panic("runtime: addSleepTask: expected next task to be nil")
 		}
 	}
-	t.Data = uint64(duration)
+	t.Data = uint(duration) // TODO: longer durations
 	now := ticks()
 	if sleepQueue == nil {
 		scheduleLog("  -> sleep new queue")