workqueue: Move workqueue_set_unbound_cpumask() and its helpers inside CONFIG_SYSFS

Commit fe28f631 ("workqueue: Add workqueue_unbound_exclude_cpumask()
to exclude CPUs from wq_unbound_cpumask") makes
workqueue_set_unbound_cpumask() static as it is not used elsewhere in
the kernel. However, this triggers a kernel test robot warning about
'workqueue_set_unbound_cpumask' defined but not used when CONFIG_SYS
isn't defined. It happens that workqueue_set_unbound_cpumask() is only
called when CONFIG_SYS is defined.

Move workqueue_set_unbound_cpumask() and its helpers inside the
CONFIG_SYSFS compilation block to avoid the warning. There is no
functional change.

Fixes: fe28f631

 ("workqueue: Add workqueue_unbound_exclude_cpumask() to exclude CPUs from wq_unbound_cpumask")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311130831.uh0AoCd1-lkp@intel.com/
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>