cpuset: fix the WARN_ON() in update_nodemasks_hier()

The WARN_ON() is used to check if we break the legal hierarchy, on
which the effective mems should be equal to configured mems.

Reported-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Tested-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>