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>

include/linux/workqueue.h

@@ -62,7 +62,7 @@ enum {
 	WORK_CPU_UNBOUND	= NR_CPUS,
 
 	/*
-	 * Reserve 7 bits off of pwq pointer w/ debugobjects turned off.
+	 * Reserve 8 bits off of pwq pointer w/ debugobjects turned off.
 	 * This makes pwqs aligned to 256 bytes and allows 15 workqueue
 	 * flush colors.
 	 */