[SPARK-18119][SPARK-CORE] Namenode safemode check is only performed o…

…n one namenode which can stuck the startup of SparkHistory server

## What changes were proposed in this pull request?

Instead of using the setSafeMode method that check the first namenode used the one which permitts to check only for active NNs
## How was this patch tested?

manual tests

Please review https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark before opening a pull request.

This commit is contributed by Criteo SA under the Apache v2 licence.

Author: n.fraison <[email protected]>

Closes apache#15648 from ashangit/SPARK-18119.

core/src/main/scala/org/apache/spark/deploy/history/FsHistoryProvider.scala

@@ -663,9 +663,9 @@ private[history] class FsHistoryProvider(conf: SparkConf, clock: Clock)
       false
   }
 
-  // For testing.
   private[history] def isFsInSafeMode(dfs: DistributedFileSystem): Boolean = {
-    dfs.setSafeMode(HdfsConstants.SafeModeAction.SAFEMODE_GET)
+    /* true to check only for Active NNs status */
+    dfs.setSafeMode(HdfsConstants.SafeModeAction.SAFEMODE_GET, true)
   }
 
   /**