Revert HDFS-8135 in branch-2.

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -4,8 +4,6 @@ Release 2.8.0 - UNRELEASED
 
   INCOMPATIBLE CHANGES
 
-    HDFS-8135. Remove the deprecated FSConstants class. (Li Lu via wheat9)
-
   NEW FEATURES
 
     HDFS-7891. A block placement policy with best rack failure tolerance.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/protocol/FSConstants.java

@@ -0,0 +1,27 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hdfs.protocol;
+
+/**
+ * @deprecated Please use {@link HdfsConstants}. This class
+ * is left only for other ecosystem projects which depended on
+ * it for SafemodeAction and DatanodeReport types.
+ */
+@Deprecated
+public abstract class FSConstants extends HdfsConstants {
+}