avoid updating index type during iterator creation (#5288)

Summary:
Right now there is a potential race condition where two threads are created to iterate through the DB (https://gist.github.com/miasantreble/88f5798a397ee7cb8e7baff9db2d9e85).  The problem is that in `BlockBasedTable::NewIndexIterator`, if both threads failed to find index_reader from block cache, they will call `CreateIndexReader->UpdateIndexType()` which creates a race to update `index_type` in the shared rep_ object. By checking the code, we realize the index type is always populated by `PrefetchIndexAndFilterBlocks` during the table `Open` call, so there is no need to update index type every time during iterator creation. This PR attempts to fix the race condition by removing the unnecessary call to `UpdateIndexType`
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5288

Differential Revision: D15252509

Pulled By: miasantreble

fbshipit-source-id: 6e3258652121d5c76d267f7ac457e15c5e84756e

table/block_based_table_reader.cc

@@ -3178,7 +3178,7 @@ BlockBasedTableOptions::IndexType BlockBasedTable::UpdateIndexType() {
 Status BlockBasedTable::CreateIndexReader(
     FilePrefetchBuffer* prefetch_buffer, IndexReader** index_reader,
     InternalIterator* preloaded_meta_index_iter, int level) {
-  auto index_type_on_file = UpdateIndexType();
+  auto index_type_on_file = rep_->index_type;
 
   auto file = rep_->file.get();
   const InternalKeyComparator* icomparator = &rep_->internal_comparator;