CleanupIteratorState() only to initialize DeletionState when super version cleanup needed

Summary:
Two changes:
1. DeletionState is only constructed when cleaning up is needed
2. Fix the bug of deletion state construction bug. A change was made in a previous patch: https://reviews.facebook.net/rROCKSDB774ed89c2405ee058086b099cbc8b29e243739cc#71a34e2e However, it somehow got lost when merging

Test Plan: make all check

Reviewers: kailiu, haobo, igor

Reviewed By: igor

CC: igor, dhruba, i.am.jin.lei, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16233

db/db_impl.cc

@@ -2743,11 +2743,11 @@ struct IterState {
 
 static void CleanupIteratorState(void* arg1, void* arg2) {
   IterState* state = reinterpret_cast<IterState*>(arg1);
-  DBImpl::DeletionState deletion_state(state->db->GetOptions().
-                                       max_write_buffer_number);
 
   bool need_cleanup = state->super_version->Unref();
   if (need_cleanup) {
+    DBImpl::DeletionState deletion_state;
+
     state->mu->Lock();
     state->super_version->Cleanup();
     state->db->FindObsoleteFiles(deletion_state, false, true);