Fix write_callback_test compile error

Summary:
Rename shadow variable name db_impl.

Fixing #3227
Closes https://github.com/facebook/rocksdb/pull/3235

Differential Revision: D6504051

Pulled By: yiwu-arbug

fbshipit-source-id: 186c9378dabb11f8d6db56f45c95cc3b029fcb88

db/write_callback_test.cc

@@ -294,7 +294,8 @@ TEST_F(WriteCallbackTest, WriteWithCallbackTest) {
                 if (seq_per_batch && two_queues) {
                   class PublishSeqCallback : public PreReleaseCallback {
                    public:
-                    PublishSeqCallback(DBImpl* db_impl) : db_impl_(db_impl) {}
+                    PublishSeqCallback(DBImpl* db_impl_in)
+                        : db_impl_(db_impl_in) {}
                     virtual Status Callback(SequenceNumber last_seq) {
                       db_impl_->SetLastPublishedSequence(last_seq);
                       return Status::OK();