Summary:
This was a peformance regression caused by https://reviews.facebook.net/D6729.
The default value of max_grandparent_overlap_factor was erroneously
set to 0 in db_bench.

This was causing compactions to create really really small files because the max_grandparent_overlap_factor was erroneously set to zero in the benchmark.

Test Plan: Run --benchmarks=overwrite

Reviewers: heyongqiang, emayanke, sheki, MarkCallaghan

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7797