Fixed a possible bug in DoCompactionWork

Summary:
There are two statuses in DoCompactionWork could be hidden.
This path fix this bug.

This seems to be a bug since CompactionFilterV2 is introduced:
https://github.com/facebook/rocksdb/blame/3.6.fb/db/db_impl.cc#L3294

Test Plan: make

Reviewers: nkg-, igor, sdong

Reviewed By: igor, sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28989