Summary:
Clang complain about an cast from uint64_t to int32 in db_stress. Fixing it.
Closes https://github.com/facebook/rocksdb/pull/2755

Differential Revision: D5655947

Pulled By: yiwu-arbug

fbshipit-source-id: cfac10e796e0adfef4727090b50975b0d6e2c9be