[SPARK-8098] [WEBUI] Show correct length of bytes on log page

The log page should only show desired length of bytes. Currently it shows bytes from the startIndex to the end of the file. The "Next" button on the page is always disabled.

Author: Carson Wang <carson.wang@intel.com>

Closes #6640 from carsonwang/logpage and squashes the following commits:

58cb3fd [Carson Wang] Show correct length of bytes on log page

core/src/main/scala/org/apache/spark/deploy/worker/ui/LogPage.scala

@@ -159,7 +159,7 @@ private[ui] class LogPage(parent: WorkerWebUI) extends WebUIPage("logPage") with
           offset
         }
       }
-      val endIndex = math.min(startIndex + totalLength, totalLength)
+      val endIndex = math.min(startIndex + byteLength, totalLength)
       logDebug(s"Getting log from $startIndex to $endIndex")
       val logText = Utils.offsetBytes(files, startIndex, endIndex)
       logDebug(s"Got log of length ${logText.length} bytes")