8259962: Shenandoah: task queue statistics is inconsistent after JDK-8255019

Reviewed-by: zgu

Author: shipilev

src/hotspot/share/gc/shenandoah/shenandoahSTWMark.cpp

@@ -79,6 +79,8 @@ void ShenandoahSTWMark::mark() {
   uint nworkers = heap->workers()->active_workers();
   task_queues()->reserve(nworkers);
 
+  TASKQUEUE_STATS_ONLY(task_queues()->reset_taskqueue_stats());
+
   {
     // Mark
     StrongRootsScope scope(nworkers);