Fix for unchecked status in CancelAllBackgroundWork (facebook#11699)

Summary:
## Summary
PR facebook#11497 introduced this. Status from `CancelPeriodicTaskScheduler()` is unchecked and causing test failure like https://app.circleci.com/pipelines/github/facebook/rocksdb/30743/workflows/24443a9b-6fc3-41e6-86c1-992d766eb1ec/jobs/642419

Pull Request resolved: facebook#11699

Test Plan: Existing tests

Reviewed By: cbi42

Differential Revision: D48287188

Pulled By: jaykorean

fbshipit-source-id: b6bcf6e3c3c47f126c34c24a3dfed2649635cc8c

db/db_impl/db_impl.cc

@@ -498,13 +498,14 @@ void DBImpl::WaitForBackgroundWork() {
 void DBImpl::CancelAllBackgroundWork(bool wait) {
   ROCKS_LOG_INFO(immutable_db_options_.info_log,
                  "Shutdown: canceling all background work");
-  CancelPeriodicTaskScheduler();
+  Status s = CancelPeriodicTaskScheduler();
+  s.PermitUncheckedError();
+
   InstrumentedMutexLock l(&mutex_);
   if (!shutting_down_.load(std::memory_order_acquire) &&
       has_unpersisted_data_.load(std::memory_order_relaxed) &&
       !mutable_db_options_.avoid_flush_during_shutdown) {
-    Status s =
-        DBImpl::FlushAllColumnFamilies(FlushOptions(), FlushReason::kShutDown);
+    s = DBImpl::FlushAllColumnFamilies(FlushOptions(), FlushReason::kShutDown);
     s.PermitUncheckedError();  //**TODO: What to do on error?
   }