Fix edge case where DatanodeAdminDefaultMonitor doubly enqueues a DatanodeDescriptor on exception

As part of the fix merged in: https://issues.apache.org/jira/browse/HDFS-16303

There was a rare edge case noticed in DatanodeAdminDefaultMonitor which causes a DatanodeDescriptor to be added twice to the pendingNodes queue. 

• a [datanode is unhealthy so it gets added to "unhealthyDns"](https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminDefaultMonitor.java#L227)
• an exception is thrown which causes [this catch block](https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminDefaultMonitor.java#L271) to execute
• the [datanode is added to "pendingNodes"](https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminDefaultMonitor.java#L276)
• under certain conditions the [datanode can be added again from "unhealthyDns" to "pendingNodes" here](https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminDefaultMonitor.java#L296)

This Jira is to track the 1 line fix for this bug