YARN-6410. FSContext.scheduler should be final (Contributed by Yeliang Cang via Daniel Templeton)

(cherry picked from commit 9d278804425f0118b590d302985408e0794b88bb)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSContext.java

@@ -29,7 +29,7 @@ public class FSContext {
   private boolean preemptionEnabled = false;
   private float preemptionUtilizationThreshold;
   private FSStarvedApps starvedApps;
-  private FairScheduler scheduler;
+  private final FairScheduler scheduler;
 
   FSContext(FairScheduler scheduler) {
     this.scheduler = scheduler;