cgroup/cpuset: Fix incorrect isolated_cpus update in update_parent_effective_cpumask()

cgroup/cpuset: Fix incorrect isolated_cpus update in update_parent_effective_cpumask()

Before commit f0af1bfc27b5 ("cgroup/cpuset: Relax constraints to
partition & cpus changes"), a cpuset partition cannot be enabled if not
all the requested CPUs can be granted from the parent cpuset. After
that commit, a cpuset partition can be created even if the requested
exclusive CPUs contain CPUs not allowed its parent.  The delmask
containing exclusive CPUs to be removed from its parent wasn't
adjusted accordingly.

That is not a problem until the introduction of a new isolated_cpus
mask in commit 11e5f407b64a ("cgroup/cpuset: Keep track of CPUs in
isolated partitions") as the CPUs in the delmask may be added directly
into isolated_cpus.

As a result, isolated_cpus may incorrectly contain CPUs that are not
isolated leading to incorrect data reporting. Fix this by adjusting
the delmask to reflect the actual exclusive CPUs for the creation of
the partition.

Fixes: 11e5f407b64a ("cgroup/cpuset: Keep track of CPUs in isolated partitions")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>