Summary: Previously when running `db_bench` with large value for `num_multi_dbs` and enabled `Options::dump_malloc_stats`, we would see most CPU spent in jemalloc locking. After this PR that no longer shows up at the top of the profile. Pull Request resolved: https://github.com/facebook/rocksdb/pull/7145 Reviewed By: riversand963 Differential Revision: D22593031 Pulled By: ajkr fbshipit-source-id: 3b3fc91f93249c6afee53f59f34c487c3fc5add6