cgroup: only inspect the root task for cgns prefixes

Because we don't support nested cgroup namespaces, we can just grab the
cgns prefixes from the root cgset's prefix list. This means we only have to
query one task for its cgroup file, instead of potentially each of them.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>