rate: ensure IO issue restarts right after sleep

rate: ensure IO issue restarts right after sleep

If we have rate_ignore_thinktime set, we account the number of bytes
we would have done in the period of the sleep. However, we need
to subtract one block, otherwise we end up doing a rated sleep
right after the thinktime sleep.

Fixes: 1a9bf814 ("Add option to ignore thinktime for rated IO")
Signed-off-by: Jens Axboe <axboe@kernel.dk>