HDFS-15249 ThrottledAsyncChecker is not thread-safe. (apache#1922)

(cherry picked from commit c12ddbd)
(cherry picked from commit e9b06d9)
Change-Id: I7ad99401da2edf4b18fa57405f3e754284336239

Author: brfrn169

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/checker/ThrottledAsyncChecker.java

@@ -117,8 +117,8 @@ public ThrottledAsyncChecker(final Timer timer,
    * will receive the same Future.
    */
   @Override
-  public Optional<ListenableFuture<V>> schedule(Checkable<K, V> target,
-                                                K context) {
+  public synchronized Optional<ListenableFuture<V>> schedule(
+      Checkable<K, V> target, K context) {
     if (checksInProgress.containsKey(target)) {
       return Optional.empty();
     }