blktests: add cgroup2 infrastructure

blktests: add cgroup2 infrastructure

In order to test io.latency and other cgroup related things we need some
supporting helpers to setup and tear down cgroup2.  This adds support
for checking that we can even configure cgroup2 things, set them up if
need be, and then add the cleanup stuff to the main cleanup function so
everything is always in a clean state.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[Omar: split into separate file, fix shellcheck errors, rework
 cleanup/exit]
Signed-off-by: Omar Sandoval <osandov@fb.com>