workqueue: Simplify a pr_warn() call in wq_select_unbound_cpu()

Use pr_warn_once() to achieve the same thing. It's simpler.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>