Add support for doing total latency percentiles

Add support for doing total latency percentiles

By default, fio does completion latency percentiles. Sometimes
what you want is total IO latency percentiles, including the
submission part as well. If that's your thing, then set

lat_percentiles=1

and get that instead of the default completion latency percentiles.

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