db_bench: --batch_size used for write benchmarks too

Summary: Now --bench_size is only used in multireadrandom tests, although the codes allow it to run in all write tests. I don't see a reason why we can't enable it.

Test Plan:
Run
   ./db_bench -benchmarks multirandomwrite --threads=5 -batch_size=16
and see the stats printed out in LOG to make sure batching really happened.

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D25509

db/db_bench.cc

@@ -1232,7 +1232,7 @@ class Benchmark {
       writes_ = (FLAGS_writes < 0 ? FLAGS_num : FLAGS_writes);
       value_size_ = FLAGS_value_size;
       key_size_ = FLAGS_key_size;
-      entries_per_batch_ = 1;
+      entries_per_batch_ = FLAGS_batch_size;
       write_options_ = WriteOptions();
       if (FLAGS_sync) {
         write_options_.sync = true;
@@ -1287,7 +1287,6 @@ class Benchmark {
       } else if (name == Slice("readrandomfast")) {
         method = &Benchmark::ReadRandomFast;
       } else if (name == Slice("multireadrandom")) {
-        entries_per_batch_ = FLAGS_batch_size;
         fprintf(stderr, "entries_per_batch = %" PRIi64 "\n",
                 entries_per_batch_);
         method = &Benchmark::MultiReadRandom;