fio: add check rate flag

fio: add check rate flag

One of the things we do a lot for each IO, is check if we should be
checking the rate. Add a specific flag for that case, so we can answer
that question very cheaply.

This is good for more than a 5% speedup for a null engine test case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>