Summary:
995fcf75 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