iolog: prevent early entry from skewing entire logging run

iolog: prevent early entry from skewing entire logging run

If we go out of ramp time, our wakeup trigger could cause us to enter
too early in add_log_sample(). If that happens, the time diff could be
negative (so huge unsigned). Since we carry that forward, we end up
logging every entry and not just over the average window.

Signed-off-by: Jens Axboe <axboe@fb.com>