8266432: ZGC: GC allocation stalls can trigger deadlocks

Reviewed-by: pliden, ayang

Author: stefank

src/hotspot/share/gc/z/zForwarding.cpp

@@ -132,12 +132,8 @@ void ZForwarding::release_page() {
 
 bool ZForwarding::wait_page_released() const {
   if (Atomic::load_acquire(&_ref_count) != 0) {
-    ZLocker<ZConditionLock> locker(&_ref_lock);
-    if (_ref_abort) {
-      return false;
-    }
-
     ZStatTimer timer(ZCriticalPhaseRelocationStall);
+    ZLocker<ZConditionLock> locker(&_ref_lock);
     while (Atomic::load_acquire(&_ref_count) != 0) {
       if (_ref_abort) {
         return false;