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

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

@@ -117,8 +117,8 @@ public class ThrottledAsyncChecker<K, V> implements AsyncChecker<K, V> {
    * 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();
     }