add GetRootDB() in DeleteFilesInRange

Summary:
In case users cast a subclass of db* into dbimpl*
Closes https://github.com/facebook/rocksdb/pull/2222

Differential Revision: D4964486

Pulled By: lightmark

fbshipit-source-id: 0ccdc08ee8e7a193dfbbe0218c3cbfd795662ca1

db/convenience.cc

@@ -21,7 +21,7 @@ void CancelAllBackgroundWork(DB* db, bool wait) {
 
 Status DeleteFilesInRange(DB* db, ColumnFamilyHandle* column_family,
                           const Slice* begin, const Slice* end) {
-  return (dynamic_cast<DBImpl*>(db))
+  return (dynamic_cast<DBImpl*>(db->GetRootDB()))
       ->DeleteFilesInRange(column_family, begin, end);
 }