db_bench: set thread pool size according to max_background_flushes

Summary: option max_background_flushes doesn't make sense if thread pool size is not set accordingly. Set the thread pool size as what we do for max_background_compactions.

Test Plan: Run db_bench with max_background_flushes > 1

Reviewers: yhchiang, igor, rven, ljin

Reviewed By: ljin

Subscribers: MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D24717

db/db_bench.cc

@@ -2904,6 +2904,9 @@ int main(int argc, char** argv) {
   // The number of background threads should be at least as much the
   // max number of concurrent compactions.
   FLAGS_env->SetBackgroundThreads(FLAGS_max_background_compactions);
+  FLAGS_env->SetBackgroundThreads(FLAGS_max_background_flushes,
+                                  rocksdb::Env::Priority::HIGH);
+
   // Choose a location for the test database if none given with --db=<path>
   if (FLAGS_db.empty()) {
     std::string default_db_path;