can_unlock set but not used
can_unlock set but not used

Test Plan: I couldn't repro, but I hope this fixes it. See the error here: https://evergreen.mongodb.com/task_log_raw/mongodb_mongo_master_ubuntu1404_rocksdb_compile_6e9fd902d5cb25aef992363efa128640affd5196_16_02_11_04_33_37/0?type=T

Reviewers: yhchiang, andrewkr, sdong, anthony

Reviewed By: anthony

Subscribers: meyering, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54123

wat-ze-hex/rocksdb,ryneli/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,facebook/rocksdb,jalexanderqed/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,Andymic/rocksdb,wenduo/rocksdb,facebook/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,hobinyoon/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,flabby/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,facebook/rocksdb,Andymic/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,facebook/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,bbiao/rocksdb,norton/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,facebook/rocksdb,jalexanderqed/rocksdb,norton/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,norton/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,flabby/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,wenduo/rocksdb,ryneli/rocksdb,ryneli/rocksdb,norton/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,hobinyoon/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,wenduo/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,norton/rocksdb,hobinyoon/rocksdb,tsheasha/rocksdb,SunguckLee/RocksDB,norton/rocksdb,Andymic/rocksdb,facebook/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,flabby/rocksdb,Andymic/rocksdb,flabby/rocksdb,jalexanderqed/rocksdb,wenduo/rocksdb,vmx/rocksdb,norton/rocksdb,OverlordQ/rocksdb,norton/rocksdb,ryneli/rocksdb