cpuset: fix missing error return in update_cpumask

The commit c6366739 ("cpuset: refactor cpus_allowed_validate_change")
inadvertently removed the error return when cpus_allowed_validate_change()
fails. This patch restores the proper error handling by returning retval
when the validation check fails.

Fixes: c6366739 ("cpuset: refactor cpus_allowed_validate_change")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>