workqueue_lock will be used to synchronize areas which require
irq-safety and there isn't much benefit in keeping it not irq-safe.
Make it irq-safe.

This patch doesn't introduce any visible behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>