stat: print the right percentile variant for json output

stat: print the right percentile variant for json output

We always show the clat percentiles for the json output, regardless
of whether lat_percentiles or clat_percentiles was defined in the
job. Correct that so we show the right output depending on the
job setting.

Fixes: https://github.com/axboe/fio/issues/691
Signed-off-by: Jens Axboe <axboe@kernel.dk>