Direct I/O Close() shouldn't rewrite the last block (facebook#4771)

Summary:
In Direct I/O case, WritableFileWriter::Close() rewrites the last block again, even if there is nothing new. The reason is that, Close() flushes the buffer. For non-direct I/O case, the buffer is empty in this case so it is a no-op. However, in direct I/O case, the partial data in the last block is kept in the buffer because it needs to be rewritten for the next write. This piece of data is flushed again. This commit fixes it by skipping this write out if `pending_sync_` flag shows that there isn't new data sync last sync.
Pull Request resolved: facebook#4771

Differential Revision: D13420426

Pulled By: siying

fbshipit-source-id: 9d39ec9a215b1425d4ed40d85e0eba1f5daa75c6

util/file_reader_writer.cc

@@ -327,7 +327,9 @@ Status WritableFileWriter::Flush() {
   if (buf_.CurrentSize() > 0) {
     if (use_direct_io()) {
 #ifndef ROCKSDB_LITE
-      s = WriteDirect();
+      if (pending_sync_) {
+        s = WriteDirect();
+      }
 #endif  // !ROCKSDB_LITE
     } else {
       s = WriteBuffered(buf_.BufferStart(), buf_.CurrentSize());