fio: work with cgroup2 as well

fio: work with cgroup2 as well

The current code only works with cgroupv1, but as long as you don't want
to set blkio.weight or anything we can use the 'cgroup=' setting with
cgroupv2 easily with cgroupv1, we just need to write the thread pids to
a different file for cgroup2.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>