Improve IOPs 50% by avoiding clock sampling when rate options not used

Improve IOPs 50% by avoiding clock sampling when rate options not used

Profiling revealed thread_main() is spending 50% of its time in calls to
utime_since_now() from rate_ddir(). This call is only necessary if the user
specified a rate option for the job. A conditional was added to avoid the call
if !should_check_rate(). See this link for details and profiling data:

https://github.com/axboe/fio/issues/1501#issuecomment-1418327049

Signed-off-by: Adam Horshack (horshack@live.com)