workqueue: Restrict kworker in the offline CPU pool running on housekeeping CPUs

When a CPU is going offline, all workers on the CPU's pool will have their
cpus_allowed cleared to cpu_possible_mask and can run on any CPUs including
the isolated ones. Instead, set cpus_allowed to wq_unbound_cpumask so that
the can avoid isolated CPUs.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>