workqueue: restore CPU affinity of unbound workers on CPU_ONLINE

With the recent addition of the custom attributes support, unbound
pools may have allowed cpumask which isn't full.  As long as some of
CPUs in the cpumask are online, its workers will maintain cpus_allowed
as set on worker creation; however, once no online CPU is left in
cpus_allowed, the scheduler will reset cpus_allowed of any workers
which get scheduled so that they can execute.

To remain compliant to the user-specified configuration, CPU affinity
needs to be restored when a CPU becomes online for an unbound pool
which doesn't currently have any online CPUs before.

This patch implement restore_unbound_workers_cpumask(), which is
called from CPU_ONLINE for all unbound pools, checks whether the
coming up CPU is the first allowed online one, and, if so, invokes
set_cpus_allowed_ptr() with the configured cpumask on all workers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>