block: don't assume device has a request list backing in nr_requests store

Stacked devices do not. For now, just error out with -EINVAL. Later
we could make the limit apply on stacked devices too, for throttling
reasons.

This fixes

5a54cd13353bb3b88887604e2c980aa01e314309

and should go into 2.6.31 stable as well.

Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-sysfs.c

@@ -40,7 +40,12 @@ queue_requests_store(struct request_queue *q, const char *page, size_t count)
 {
 	struct request_list *rl = &q->rq;
 	unsigned long nr;
-	int ret = queue_var_store(&nr, page, count);
+	int ret;
+
+	if (!q->request_fn)
+		return -EINVAL;
+
+	ret = queue_var_store(&nr, page, count);
 	if (nr < BLKDEV_MIN_RQ)
 		nr = BLKDEV_MIN_RQ;