HDFS-8051. Fix missing import statement from backport.

Author: Colin Patrick Mccabe

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsVolumeList.java

@@ -40,6 +40,7 @@
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.VolumeChoosingPolicy;
 import org.apache.hadoop.hdfs.server.datanode.BlockScanner;
 import org.apache.hadoop.hdfs.server.protocol.DatanodeStorage;
+import org.apache.hadoop.io.IOUtils;
 import org.apache.hadoop.util.DiskChecker.DiskErrorException;
 import org.apache.hadoop.util.Time;