HDFS-13195. DataNode conf page cannot display the current value after reconfig. Contributed by maobaolong.

(cherry picked from commit 66461ed98b0b2f630b682bff927dbd74c251c26e)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/web/DatanodeHttpServer.java

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/web/DatanodeHttpServer.java

@@ -121,6 +121,7 @@ public class DatanodeHttpServer implements Closeable {
     builder.configureXFrame(xFrameEnabled).setXFrameOption(xFrameOptionValue);
 
     this.infoServer = builder.build();
+    this.infoServer.setAttribute(HttpServer2.CONF_CONTEXT_ATTRIBUTE, conf);
 
     this.infoServer.addInternalServlet(null, "/streamFile/*", StreamFile.class);
     this.infoServer.addInternalServlet(null, "/getFileChecksum/*",