add use_direct_io() to ReadaheadRandomAccessFile

Summary:
Missing this function will cause RandomAccessFileReader not doing alignment in Direct IO mode, which introduce an IOError: invalid argument.
Closes https://github.com/facebook/rocksdb/pull/1900

Differential Revision: D4601261

Pulled By: lightmark

fbshipit-source-id: c3eadf1

util/file_reader_writer.cc

@@ -543,6 +543,10 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
     return file_->InvalidateCache(offset, length);
   }
 
+  virtual bool use_direct_io() const override {
+    return file_->use_direct_io();
+  }
+
  private:
   std::unique_ptr<RandomAccessFile> file_;
   size_t               readahead_size_;