cgroups: restore perms on tasks and cgroup.procs files

Although we don't want to restore the values on these files, we definitely
do want to restore the permissions, as certain container engines (lxc) make
use of this.

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