workqueue: use enum value to set array size of pools in gcwq

Commit 3270476a ('workqueue: reimplement
WQ_HIGHPRI using a separate worker_pool') introduce separate worker_pool
for HIGHPRI. Although there is NR_WORKER_POOLS enum value which represent
size of pools, definition of worker_pool in gcwq doesn't use it.
Using it makes code robust and prevent future mistakes.
So change code to use this enum value.
Signed-off-by: NJoonsoo Kim <js1304@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -183,7 +183,8 @@ struct global_cwq {
 	struct hlist_head	busy_hash[BUSY_WORKER_HASH_SIZE];
 						/* L: hash of busy workers */
 
-	struct worker_pool	pools[2];	/* normal and highpri pools */
+	struct worker_pool	pools[NR_WORKER_POOLS];
+						/* normal and highpri pools */
 
 	wait_queue_head_t	rebind_hold;	/* rebind hold wait */
 } ____cacheline_aligned_in_smp;