workqueue: fix a piece of comment about reserved bits for work flags

8a2e8e5d("workqueue: fix cwq->nr_active underflow")
allocated one more bit from the work flags, and it updated
partial of the comments (128 bytes -> 256 bytes), but it
failed to update the info about the number of reserved bits.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Tejun Heo <tj@kernel.org>