call GetRootDB() before cast to DBImpl* in CancelAllBackgroundWork

Summary:
User could call this with wrapper class of DB or DBImpl
Closes https://github.com/facebook/rocksdb/pull/2200

Differential Revision: D4935530

Pulled By: lightmark

fbshipit-source-id: df9cb61d67d0f3bbcf62f714d77523a459a92883

db/convenience.cc

@@ -16,7 +16,7 @@
 namespace rocksdb {
 
 void CancelAllBackgroundWork(DB* db, bool wait) {
-  (dynamic_cast<DBImpl*>(db))->CancelAllBackgroundWork(wait);
+  (dynamic_cast<DBImpl*>(db->GetRootDB()))->CancelAllBackgroundWork(wait);
 }
 
 Status DeleteFilesInRange(DB* db, ColumnFamilyHandle* column_family,