Fix TSAN failure for backupable_db_test (#7478)

Summary:
It's a transient failure, but can be reproduce with running the test 100
times:
https://app.circleci.com/pipelines/github/facebook/rocksdb/3760/workflows/de909685-f22b-45ba-a8f3-6ebb78a54e96/jobs/37039

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7478

Test Plan: re-run the test 100 times

Reviewed By: ajkr

Differential Revision: D24035758

Pulled By: jay-zhuang

fbshipit-source-id: 6b31983d5c3f7faa8d5481306098513485d0d69d

utilities/backupable/backupable_db_test.cc

@@ -1423,7 +1423,7 @@ TEST_F(BackupableDBTest, TableFileWithoutDbChecksumCorruptedDuringBackup) {
                         kShareWithChecksum);
 
   FillDB(db_.get(), 0, keys_iteration);
-  bool corrupted = false;
+  std::atomic<bool> corrupted{false};
   // corrupt files when copying to the backup directory
   SyncPoint::GetInstance()->SetCallBack(
       "BackupEngineImpl::CopyOrCreateFile:CorruptionDuringBackup",