HDFS-16443. Fix edge case where DatanodeAdminDefaultMonitor doubly enqueues a DatanodeDescriptor on exception (#3942)

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>
(cherry picked from commit 089e06de2135988c326328e544f04b5129c53517)

 Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminDefaultMonitor.java

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java

@@ -636,6 +636,7 @@ public class DatanodeAdminManager {
               + "{}.", dn, e);
           pendingNodes.add(dn);
           toRemove.add(dn);
+          unhealthyDns.remove(dn);
         } finally {
           iterkey = dn;
         }