Fix a flaky tsan test for DBTest2 (#7526)

Summary:
ThreadSanitizer: data race for `DummyOldStats.num_rt`.
Failed build: https://app.circleci.com/pipelines/github/facebook/rocksdb/3991/workflows/b47c3ae1-5531-4489-ac51-11854abdfd0f/jobs/42305

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7526

Reviewed By: akankshamahajan15

Differential Revision: D24226736

Pulled By: jay-zhuang

fbshipit-source-id: e05ce354d0c0db0eba242d59d4b0e89ce7c25acf

db/db_test2.cc

@@ -4376,8 +4376,8 @@ class DummyOldStats : public Statistics {
   }
   bool HistEnabledForType(uint32_t /*type*/) const override { return false; }
   std::string ToString() const override { return ""; }
-  int num_rt = 0;
-  int num_mt = 0;
+  std::atomic<int> num_rt{0};
+  std::atomic<int> num_mt{0};
 };
 }  // namespace