fix assert failed when timeout during call rate_ddir.

fix assert failed when timeout during call rate_ddir.

Adding DDIR_TIMEOUT in enum fio_ddir, and rate_ddir returns it when fio timeouts.
set_io_u_file will directly break out of the loop, and fill_io_u won't be called,
which causes assert to fail in rate_ddir, because td->rwmix_ddir is DDIR_INVAL.

Signed-off-by: QingSong Zhu zhuqingsong.0909@bytedance.com