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

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

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

@@ -275,6 +275,7 @@ public class DatanodeAdminDefaultMonitor extends DatanodeAdminMonitorBase
             + "{}.", dn, e);
         getPendingNodes().add(dn);
         toRemove.add(dn);
+        unhealthyDns.remove(dn);
       } finally {
         iterkey = dn;
       }