Sync before closeIntoReader in Translog.rollGeneration.

Backport of elastic/elasticsearch#45765

Make sure we move most of the data to disk outside of the writeLock
in order to reduce the time the lock is held since its blocking
all thread.

es/es-server/src/main/java/org/elasticsearch/index/translog/Translog.java

@@ -1610,6 +1610,9 @@ public TranslogGeneration getMinGenerationForSeqNo(final long seqNo) {
      * @throws IOException if an I/O exception occurred during any file operations
      */
     public void rollGeneration() throws IOException {
+        // make sure we move most of the data to disk outside of the writeLock
+        // in order to reduce the time the lock is held since it's blocking all threads
+        sync();
         try (Releasable ignored = writeLock.acquire()) {
             try {
                 final TranslogReader reader = current.closeIntoReader();