fix the ns_cgroup vs clone_children

fix the ns_cgroup vs clone_children

The following patch fixes the bug where the clone_children compatibility
flag is available with the ns_cgroup subsystem. The 2.6.37 kernel version
should be the only one which is concerned by this modification, please
refer to Documentation/feature-removal-schedule.txt and look for ns_cgroup.

The problem is coming from we check for clone_children and we set it
automatically and then we try to create a new cgroup. As the
ns_cgroup is present the cgroup already exists and we are not allowed
to attach our pid to a new cgroup. The next error will be when we try
to create a new container because we enabled the clone_children flag
and the ns_cgroup is present, it is not allowed by the kernel.

The patch fix this by checking the mount options.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>