blk-mq: fix buffer overflow when reading sysfs file of 'pending'

There may be lots of pending requests so that the buffer of PAGE_SIZE
can't hold them at all.

One typical example is scsi-mq, the queue depth(.can_queue) of
scsi_host and blk-mq is quite big but scsi_device's queue_depth
is a bit small(.cmd_per_lun), then it is quite easy to have lots
of pending requests in hw queue.

This patch fixes the following warning and the related memory
destruction.

[  359.025101] fill_read_buffer: blk_mq_hw_sysfs_show+0x0/0x7d returned bad count^M
[  359.055595] irq event stamp: 15537^M
[  359.055606] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC ^M
[  359.055614] Dumping ftrace buffer:^M
[  359.055660]    (ftrace buffer empty)^M
[  359.055672] Modules linked in: nbd ipv6 kvm_intel kvm serio_raw^M
[  359.055678] CPU: 4 PID: 21631 Comm: stress-ng-sysfs Not tainted 4.2.0-rc5-next-20150805 #434^M
[  359.055679] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011^M
[  359.055682] task: ffff8802161cc000 ti: ffff88021b4a8000 task.ti: ffff88021b4a8000^M
[  359.055693] RIP: 0010:[<ffffffff811541c5>]  [<ffffffff811541c5>] __kmalloc+0xe8/0x152^M

Cc: <stable@vger.kernel.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq-sysfs.c

@@ -141,15 +141,26 @@ static ssize_t blk_mq_sysfs_completed_show(struct blk_mq_ctx *ctx, char *page)
 
 static ssize_t sysfs_list_show(char *page, struct list_head *list, char *msg)
 {
-	char *start_page = page;
 	struct request *rq;
+	int len = snprintf(page, PAGE_SIZE - 1, "%s:\n", msg);
+
+	list_for_each_entry(rq, list, queuelist) {
+		const int rq_len = 2 * sizeof(rq) + 2;
+
+		/* if the output will be truncated */
+		if (PAGE_SIZE - 1 < len + rq_len) {
+			/* backspacing if it can't hold '\t...\n' */
+			if (PAGE_SIZE - 1 < len + 5)
+				len -= rq_len;
+			len += snprintf(page + len, PAGE_SIZE - 1 - len,
+					"\t...\n");
+			break;
+		}
+		len += snprintf(page + len, PAGE_SIZE - 1 - len,
+				"\t%p\n", rq);
+	}
 
-	page += sprintf(page, "%s:\n", msg);
-
-	list_for_each_entry(rq, list, queuelist)
-		page += sprintf(page, "\t%p\n", rq);
-
-	return page - start_page;
+	return len;
 }
 
 static ssize_t blk_mq_sysfs_rq_list_show(struct blk_mq_ctx *ctx, char *page)