cgroup: don't set clone_children when it is already 1

In particular, if it's already 1, and we can't change it, we currently
fail out.  That's silly.

I was going to just always continue, but if clone_children is not 1,
then the container *will* fail to start later on, so I'd rather stop
earlier on so the original cause doesn't get lost in the noise.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>