Fix 'min' latency times being 0 with ramp_time

Fix 'min' latency times being 0 with ramp_time

If the job includes a ramp_time setting, we end up with latencies
that look like this:

    slat (nsec): min=0, max=17585, avg=1896.34, stdev=733.35
    clat (nsec): min=0, max=1398.1k, avg=77851.76, stdev=25055.97
     lat (nsec): min=0, max=1406.1k, avg=79824.20, stdev=25066.57

with the 'min' being 0. This is because the reset stats sets the
field to zero, and no new IO will be smaller than that...

Set the min value to the max value of the type when we reset stats.

Reported-by: Matthew Eaton <m.eaton82@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>