cgroup/cpuset: Correct invalid remote parition prs

When enable a remote partition, I found that:

cd /sys/fs/cgroup/
mkdir test
mkdir test/test1
echo +cpuset > cgroup.subtree_control
echo +cpuset >  test/cgroup.subtree_control
echo 3 > test/test1/cpuset.cpus
echo root > test/test1/cpuset.cpus.partition
cat test/test1/cpuset.cpus.partition
root invalid (Parent is not a partition root)

The parent of a remote partition could not be a root. This is due to the
emtpy effective_xcpus. It would be better to prompt the message "invalid
cpu list in cpuset.cpus.exclusive".

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>