Fix that an canceled ScheduledTask can still be executed in rare case (#398)

Author: yyjdelete

src/DotNetty.Common/Concurrency/ScheduledTask.cs

@@ -29,7 +29,7 @@ protected ScheduledTask(AbstractScheduledEventExecutor executor, PreciseTimeSpan
 
         public bool Cancel()
         {
-            if (!this.AtomicCancellationStateUpdate(CancellationProhibited, CancellationRequested))
+            if (!this.AtomicCancellationStateUpdate(CancellationRequested, CancellationProhibited))
             {
                 return false;
             }