DBImpl::IngestExternalFile() should grab mutex when releasing file number in failure case (#4189)

Summary:
995fcf7573 has a bug: ReleaseFileNumberFromPendingOutputs() added is not protected by the DB mutex. Fix it by grabbing the lock for this operation.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4189

Differential Revision: D9015447

Pulled By: siying

fbshipit-source-id: b8506e09a96c3f95a6fe32b5ca5fcdb9bee88937

db/db_impl.cc

@@ -2912,6 +2912,7 @@ Status DBImpl::IngestExternalFile(
   status = ingestion_job.Prepare(external_files, super_version);
   CleanupSuperVersion(super_version);
   if (!status.ok()) {
+    InstrumentedMutexLock l(&mutex_);
     ReleaseFileNumberFromPendingOutputs(pending_output_elem);
     return status;
   }