HDFS-17035. FsVolumeImpl#getActualNonDfsUsed may return negative value. (#5708). Contributed by farmmamba.

Reviewed-by: Shuyan Zhang <zqingchai@gmail.com>
Reviewed-by: He Xiaoqiao <hexiaoqiao@apache.org>
Signed-off-by: Ayush Saxena <ayushsaxena@apache.org>

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

@@ -499,7 +499,7 @@ public class FsVolumeImpl implements FsVolumeSpi {
         return used;
       }
     }
-    return getDfUsed() - getDfsUsed();
+    return Math.max(getDfUsed() - getDfsUsed(), 0L);
   }
 
   /**