block: fix oops on !disk->queue and sysfs discard alignment display

Eric Dumazet reports:

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At boot, I have a crash in part_discard_alignment_show+0x1b/0x50

CR2 : 000006ac

fault in : mov    0x2c(%rcx),%edx

I suspect commit 23ceb5b7 (block: Remove extra
discard_alignment from hd_struct) being in fault

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Not quite known how ->queue can be NULL while the sysfs entry
exists, but lets play it safe and check for a NULL queue.
The rest of the sysfs show strategies in check.c do not dereference
disk->queue.
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/partitions/check.c

@@ -256,10 +256,12 @@ ssize_t part_discard_alignment_show(struct device *dev,
 {
 	struct hd_struct *p = dev_to_part(dev);
 	struct gendisk *disk = dev_to_disk(dev);
+	unsigned int alignment = 0;
 
-	return sprintf(buf, "%u\n",
-			queue_limit_discard_alignment(&disk->queue->limits,
-							p->start_sect));
+	if (disk->queue)
+		alignment = queue_limit_discard_alignment(&disk->queue->limits,
+								p->start_sect);
+	return sprintf(buf, "%u\n", alignment);
 }
 
 ssize_t part_stat_show(struct device *dev,