During boot, depending on how the housekeeping and workqueue.unbound_cpus
masks are set, wq_unbound_cpumask can end up empty. Since 8639eceb
("workqueue: Implement non-strict affinity scope for unbound workqueues"),
this may end up feeding -1 as a CPU number into scheduler leading to oopses.

  BUG: unable to handle page fault for address: ffffffff8305e9c0
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  ...
  Call Trace:
   <TASK>
   select_idle_sibling+0x79/0xaf0
   select_task_rq_fair+0x1cb/0x7b0
   try_to_wake_up+0x29c/0x5c0
   wake_up_process+0x19/0x20
   kick_pool+0x5e/0xb0
   __queue_work+0x119/0x430
   queue_work_on+0x29/0x30
  ...

An empty wq_unbound_cpumask is a clear misconfiguration and already
disallowed once system is booted up. Let's warn on and ignore
unbound_cpumask restrictions which lead to no unbound cpus. While at it,
also remove now unncessary empty check on wq_unbound_cpumask in
wq_select_unbound_cpu().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-Tested-by: Yong He <alexyonghe@tencent.com>
Link: http://lkml.kernel.org/r/20231120121623.119780-1-alexyonghe@tencent.com
Fixes: 8639eceb ("workqueue: Implement non-strict affinity scope for unbound workqueues")
Cc: stable@vger.kernel.org # v6.6+
Reviewed-by: Waiman Long <longman@redhat.com>