cgroupv2: fix fs2 driver default path
cgroupv2: fix fs2 driver default path

When the cgroupv2 fs driver is used without setting cgroupsPath,
it picks up a path from /proc/self/cgroup. On a host with systemd,
such a path can look like (examples from my machines):

 - /user.slice/user-1000.slice/session-4.scope
 - /user.slice/user-1000.slice/user@1000.service/gnome-launched-xfce4-terminal.desktop-4260.scope
 - /user.slice/user-1000.slice/user@1000.service/gnome-terminal-server.service

This cgroup already contains processes in it, which prevents to enable
controllers for a sub-cgroup (writing to cgroup.subtree_control fails
with EBUSY or EOPNOTSUPP).

Obviously, a parent cgroup (which does not contain tasks) should be used.

Fixes opencontainers/runc/issues/2298

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

xiaochenshen/runc,mikebrow/runc,kolyshkin/runc,dqminh/runc,opencontainers/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,estesp/runc,mikebrow/runc,mikebrow/runc,xiaochenshen/runc,estesp/runc,rhatdan/runc,dqminh/runc,rhatdan/runc,kolyshkin/runc,rhatdan/runc,xiaochenshen/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,estesp/runc,opencontainers/runc,dqminh/runc