LUCENE-3100: IW.commit() writes but fails to fsync the N.fnx file

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1104090 13f79535-47bb-0310-9956-ffa450edef68

lucene/src/java/org/apache/lucene/index/SegmentInfos.java

@@ -831,6 +831,10 @@ public final class SegmentInfos extends Vector<SegmentInfo> {
         } catch (Throwable t) {
           // throw orig excp
         }
+      } else {
+        // we must sync here explicitly since during a commit
+        // IW will not sync the global field map. 
+        dir.sync(Collections.singleton(name));
       }
     }
     return version;