cgroup/cpuset: Add test cases for sibling CPU exclusion on partition update

cgroup/cpuset: Add test cases for sibling CPU exclusion on partition update

When sibling CPU exclusion occurs, a partition's effective_xcpus may be
a subset of its user_xcpus. The partcmd_update path must use
effective_xcpus instead of user_xcpus when calculating CPUs to return
to or request from the parent.

Add two test cases to verify this behavior:

  1) Narrowing cpuset.cpus to only the sibling-excluded CPUs should not
     return CPUs to parent that the partition never actually owned.

  2) Expanding cpuset.cpus after a sibling becomes a member should
     correctly request the additional CPUs from parent.

Co-developed-by: Zhang Guopeng <zhangguopeng@kylinos.cn>
Signed-off-by: Zhang Guopeng <zhangguopeng@kylinos.cn>
Signed-off-by: Sun Shaojie <sunshaojie@kylinos.cn>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>