Single-depth latency probe fio

Pure per-IO latency at iodepth=1; reports p99/p99.9/p99.99 tail latency

fio --name=latency --filename=/dev/<id> --rw=randread --bs=4k --iodepth=1 --numjobs=1 --runtime=30 --time_based --direct=1 --percentile_list=99:99.9:99.99