cpuset: introduce partition_cpus_change

Introduce the partition_cpus_change function to handle both regular CPU
set updates and exclusive CPU modifications, either of which may trigger
partition state changes. This generalized function will also be utilized
for exclusive CPU updates in subsequent patches.

With the introduction of compute_trialcs_excpus in a previous patch,
the trialcs->effective_xcpus field is now consistently computed and
maintained. Consequently, the legacy logic which assigned
**trialcs->allowed_cpus to a local 'xcpus' variable** when
trialcs->effective_xcpus was empty has been removed.

This removal is safe because when trialcs is not a partition member,
trialcs->effective_xcpus is now correctly populated with the intersection
of user_xcpus and the parent's effective_xcpus. This calculation inherently
covers the scenario previously handled by the removed code.

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