[Performance Branch] Fix a bug when merging from master

Summary: Commit "1304d8c8cefe66be1a3caa5e93413211ba2486f2" (Merge branch 'master' into performance) removes a line in performance branch by mistake. This patch fixes it.

Test Plan: make all check

Reviewers: haobo, kailiu, igor

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15297

db/memtable.cc

@@ -95,6 +95,7 @@ class MemTableIterator: public Iterator {
     if (options.prefix) {
       iter_.reset(mem_.table_->GetPrefixIterator(*options.prefix));
     } else if (options.prefix_seek) {
+      dynamic_prefix_seek_ = true;
       iter_.reset(mem_.table_->GetDynamicPrefixIterator());
     } else {
       iter_.reset(mem_.table_->GetIterator());