raid5: Use slow_path to release stripe when mddev->thread is null

When release_stripe() is called in grow_one_stripe(), the
mddev->thread is null. So it will omit one wakeup this thread to
release stripe.
For this condition, use slow_path to release stripe.

Bug was introduced in 3.12

Cc: stable@vger.kernel.org (3.12+)
Fixes: 773ca82fa1ee58dd1bf88b
Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -340,7 +340,8 @@ static void release_stripe(struct stripe_head *sh)
 	unsigned long flags;
 	bool wakeup;
 
-	if (test_and_set_bit(STRIPE_ON_RELEASE_LIST, &sh->state))
+	if (unlikely(!conf->mddev->thread) ||
+		test_and_set_bit(STRIPE_ON_RELEASE_LIST, &sh->state))
 		goto slow_path;
 	wakeup = llist_add(&sh->release_list, &conf->released_stripes);
 	if (wakeup)