Fix a timer bug in MergingIterator::Seek() caused by #5871 (#5874)

Summary:
Conflict resolving in 846e05005d ("Revert "Merging iterator to avoid child iterator reseek for some cases") caused some timer misplaced. Fix it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5874

Test Plan: See it build.

Differential Revision: D17705073

fbshipit-source-id: 9bd3a8dc4901ac33c2c6fc5b1091ffbc56a8529f

table/merging_iterator.cc

@@ -112,16 +112,19 @@ class MergingIterator : public InternalIterator {
     ClearHeaps();
     status_ = Status::OK();
     for (auto& child : children_) {
+      {
         PERF_TIMER_GUARD(seek_child_seek_time);
-
         child.Seek(target);
+      }
 
       PERF_COUNTER_ADD(seek_child_seek_count, 1);
+      {
         // Strictly, we timed slightly more than min heap operation,
         // but these operations are very cheap.
         PERF_TIMER_GUARD(seek_min_heap_time);
         AddToMinHeapOrCheckStatus(&child);
       }
+    }
     direction_ = kForward;
     {
       PERF_TIMER_GUARD(seek_min_heap_time);