cgroups: handle offline cpus in v1 hierarchy
cgroups: handle offline cpus in v1 hierarchy

Handle offline cpus in v1 hierarchy.

In addition to isolated cpus we also need to account for offline cpus when our
ancestor cgroup is the root cgroup and we have not been initialized yet.

Closes #2953.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

ss1h2a3tw/lxc,hallyn/lxc,hallyn/lxc,ss1h2a3tw/lxc,hallyn/lxc,ss1h2a3tw/lxc