cgroup: Fix early update of @dir_name in rewrite_cgsets

When we're walking over controllers and their directories
we should update the directory at the very end because
otherwise we will not match the rest of them once variable
is updated.

So I desided to simply rewrite the helper. Hopefully it
is more clear now.

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