Limit key range to number of keys, not number of writes

Summary:
An old commit (482401) changed DoWrite to use the value of --writes rather
than --num to determine the range for keys. This restores the old and correct
behavior which is to limit it using --num.

Task ID: #6353043

Blame Rev:

Test Plan:
run db_bench

Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D34065

db/db_bench.cc

@@ -2266,7 +2266,7 @@ class Benchmark {
 
     Duration duration(test_duration, max_ops, ops_per_stage);
     for (size_t i = 0; i < num_key_gens; i++) {
-      key_gens[i].reset(new KeyGenerator(&(thread->rand), write_mode, num_ops,
+      key_gens[i].reset(new KeyGenerator(&(thread->rand), write_mode, num_,
                                          ops_per_stage));
     }