cgroup/cpuset: Return only actually allocated CPUs during partition invalidation

In update_parent_effective_cpumask() with partcmd_invalidate, the CPUs
to return to the parent are computed as:

    adding = cpumask_and(tmp->addmask, xcpus, parent->effective_xcpus);

where xcpus = user_xcpus(cs) which returns cs->exclusive_cpus (if set)
or cs->cpus_allowed. When exclusive_cpus is not set, user_xcpus(cs) can
contain CPUs that were never actually granted to the partition due to
sibling exclusion in compute_excpus(). Consequently, the invalidation
may return CPUs to the parent that remain in use by sibling partitions,
causing overlapping effective_cpus and triggering the
WARN_ON_ONCE(1) in generate_sched_domains().

Use cs->effective_xcpus instead, which reflects the CPUs actually
granted to this partition.

Reproducer (on a 4-CPU machine):

    cd /sys/fs/cgroup
    mkdir a1 b1

    # a1 becomes partition root with CPUs 0-1
    echo "0-1" > a1/cpuset.cpus
    echo "root" > a1/cpuset.cpus.partition

    # b1 becomes partition root with CPUs 1-2, but sibling exclusion
    # reduces its effective_xcpus to CPU 2 only
    echo "1-2" > b1/cpuset.cpus
    echo "root" > b1/cpuset.cpus.partition

    # b1 changes cpus_allowed to 0-1 -> partition invalidation
    echo "0-1" > b1/cpuset.cpus

    # Expected: CPUs 2-3  (only CPU 2 returned from b1)
    # Actual:   CPUs 1-3  (CPU 0-1 returned, overlapping with a1)
    cat cpuset.cpus.effective

dmesg will also show a WARNING from generate_sched_domains() reporting
overlapping partition root effective_cpus.

Fixes: 2a360203 ("cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict")
Cc: stable@vger.kernel.org # v7.0+
Signed-off-by: sunshaojie <sunshaojie@kylinos.cn>
Tested-by: Chen Ridong <chenridong@huaweicloud.com>
Reviewed-by: Chen Ridong <chenridong@huaweicloud.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>