db_bench should use the default value for max_grandparent_overlap_factor.

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

db/db_bench.cc

@@ -219,7 +219,7 @@ static double FLAGS_rate_limit = 0;
 
 // Control maximum bytes of overlaps in grandparent (i.e., level+2) before we
 // stop building a single file in a level->level+1 compaction.
-static int FLAGS_max_grandparent_overlap_factor;
+static int FLAGS_max_grandparent_overlap_factor = 10;
 
 // Run read only benchmarks.
 static bool FLAGS_read_only = false;