cpuset: fix failure to enable isolated partition when containing isolcpus

The 'isolcpus' parameter specified at boot time can be assigned to an
isolated partition. While it is valid put the 'isolcpus' in an isolated
partition, attempting to change a member cpuset to an isolated partition
will fail if the cpuset contains any 'isolcpus'.

For example, the system boots with 'isolcpus=9', and the following
configuration works correctly:

  # cd /sys/fs/cgroup/
  # mkdir test
  # echo 1 > test/cpuset.cpus
  # echo isolated > test/cpuset.cpus.partition
  # cat test/cpuset.cpus.partition
  isolated
  # echo 9 > test/cpuset.cpus
  # cat test/cpuset.cpus.partition
  isolated
  # cat test/cpuset.cpus
  9

However, the following steps to convert a member cpuset to an isolated
partition will fail:

  # cd /sys/fs/cgroup/
  # mkdir test
  # echo 9 > test/cpuset.cpus
  # echo isolated > test/cpuset.cpus.partition
  # cat test/cpuset.cpus.partition
  isolated invalid (partition config conflicts with housekeeping setup)

The issue occurs because the new partition state (new_prs) is used for
validation against housekeeping constraints before it has been properly
updated. To resolve this, move the assignment of new_prs before the
housekeeping validation check when enabling a root partition.

Fixes: 4a74e418 ("cgroup/cpuset: Check partition conflict with housekeeping setup")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>