workqueue: Remove GPF argument from alloc_workqueue_attrs()

All callers use GFP_KERNEL. No point in having that argument.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>