Fix a uncleaned counter in PerfContext::Reset()

Summary:
new_table_iterator_nanos is not cleaned in PerfContext::Reset() while new_table_block_iter_nanos is cleaned twice. Fix it.
Also fix a comment.

Test Plan: Build and db_bench with --perf_context to see the value shown.

Reviewers: kradhakrishnan, anthony, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D41721

include/rocksdb/perf_context.h

@@ -78,7 +78,7 @@ struct PerfContext {
   uint64_t read_filter_block_nanos;
   // Time spent on creating data block iterator
   uint64_t new_table_block_iter_nanos;
-  // Time spent on new_table_block_iter_micros
+  // Time spent on creating a iterator of an SST file.
   uint64_t new_table_iterator_nanos;
   // Time spent on seeking a key in data/index blocks
   uint64_t block_seek_nanos;

util/perf_context.cc

@@ -50,7 +50,7 @@ void PerfContext::Reset() {
   read_index_block_nanos = 0;
   read_filter_block_nanos = 0;
   new_table_block_iter_nanos = 0;
-  new_table_block_iter_nanos = 0;
+  new_table_iterator_nanos = 0;
   block_seek_nanos = 0;
   find_table_nanos = 0;
 #endif