Use default value instead of hard-coded 0 for compaction_readhead_siz…

…e in db bench (facebook#11831)

Summary:
**Context/Summary:**
It allows db bench reflect the default behavior of this option. For example, we recently changed its default value.

Pull Request resolved: facebook#11831

Test Plan: No code change

Reviewed By: cbi42

Differential Revision: D49253690

Pulled By: hx235

fbshipit-source-id: 445d4e54f62b4b538626e301a3014d2f00849d30

tools/db_bench_tool.cc

@@ -718,7 +718,9 @@ DEFINE_int32(file_opening_threads,
              "If open_files is set to -1, this option set the number of "
              "threads that will be used to open files during DB::Open()");
 
-DEFINE_int32(compaction_readahead_size, 0, "Compaction readahead size");
+DEFINE_int32(compaction_readahead_size,
+             ROCKSDB_NAMESPACE::Options().compaction_readahead_size,
+             "Compaction readahead size");
 
 DEFINE_int32(log_readahead_size, 0, "WAL and manifest readahead size");