Fix initialization of lastrate when using ratemin

fio was copying td->bw_sample_time (single timeval) into td->lastrate
(array of two timevals) using the latter's size. Thus is would get one
timeval worth of good data plus who-knows-what for the second timeval.
I'm pretty sure the intent of this code was to copy bw_sample_time
into both members of lastrate.

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