Revert "block: Remove extra discard_alignment from hd_struct."

It was not a good idea to start dereferencing disk->queue from
the fs sysfs strategy for displaying discard alignment. We ran
into first a NULL pointer deref, and after fixing that we sometimes
see unvalid disk->queue pointer values.

Since discard is the only one of the bunch actually looking into
the queue, just revert the change.

This reverts commit 23ceb5b7719e9276d4fa72a3ecf94dd396755276.

Conflicts:
fs/partitions/check.c

fs/partitions/check.c

···
 				   struct device_attribute *attr, char *buf)
 {
 	struct hd_struct *p = dev_to_part(dev);
-	struct gendisk *disk = dev_to_disk(dev);
-	unsigned int alignment = 0;
-
-	if (disk->queue)
-		alignment = queue_limit_discard_alignment(&disk->queue->limits,
-								p->start_sect);
-	return sprintf(buf, "%u\n", alignment);
+	return sprintf(buf, "%u\n", p->discard_alignment);
 }
 
 ssize_t part_stat_show(struct device *dev,
···
 	p->start_sect = start;
 	p->alignment_offset =
 		queue_limit_alignment_offset(&disk->queue->limits, start);
+	p->discard_alignment =
+		queue_limit_discard_alignment(&disk->queue->limits, start);
 	p->nr_sects = len;
 	p->partno = partno;
 	p->policy = get_disk_ro(disk);

include/linux/genhd.h

···
 	sector_t start_sect;
 	sector_t nr_sects;
 	sector_t alignment_offset;
+	unsigned int discard_alignment;
 	struct device __dev;
 	struct kobject *holder_dir;
 	int policy, partno;