WritePrepared: fix race condition on GetSnapshotListFromDB (#4872)

Summary:
Fixes a typo that made mutex_ to remain unlocked when GetSnapshotListFromDB called from WritePreparedTxnDB.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4872

Differential Revision: D13640381

Pulled By: maysamyabandeh

fbshipit-source-id: 50f6600568f9092b4b43115f6ebd96e6c7388ad7

utilities/transactions/write_prepared_txn_db.cc

@@ -567,7 +567,8 @@ const Snapshot* WritePreparedTxnDB::GetSnapshot() {
 const std::vector<SequenceNumber> WritePreparedTxnDB::GetSnapshotListFromDB(
     SequenceNumber max) {
   ROCKS_LOG_DETAILS(info_log_, "GetSnapshotListFromDB with max %" PRIu64, max);
-  InstrumentedMutex(db_impl_->mutex());
+  InstrumentedMutexLock dblock(db_impl_->mutex());
+  db_impl_->mutex()->AssertHeld();
   return db_impl_->snapshots().GetAll(nullptr, max);
 }