YARN-7400. Incorrect log preview displayed in jobhistory server ui. Contributed by Xuan Gong.

(cherry picked from commit d57dba99428cbe3f3dfcec834d79f709e7529ef9)
(cherry picked from commit d1f8854a1b9ff2473c1784aa8a8bb06a61819359)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/logaggregation/filecontroller/ifile/IndexedFileAggregatedLogsBlock.java

@@ -221,8 +221,8 @@ public class IndexedFileAggregatedLogsBlock extends LogAggregationHtmlBlock {
                       _(" for the full log.")._();
             }
             long totalSkipped = 0;
-            while (totalSkipped < start) {
-              long ret = in.skip(start - totalSkipped);
+            while (totalSkipped < startIndex) {
+              long ret = in.skip(startIndex - totalSkipped);
               if (ret == 0) {
                 //Read one byte
                 int nextByte = in.read();