workqueue: Factor out pwq_is_empty()

[ Upstream commit afa87ce8 ]

"!pwq->nr_active && list_empty(&pwq->inactive_works)" test is repeated
multiple times. Let's factor it out into pwq_is_empty().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c1 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>