block: simplify part_to_disk

Now that struct hd_struct has a block_device pointer use that to
find the disk.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/genhd.h

@@ -218,13 +218,9 @@ struct gendisk {
 
 static inline struct gendisk *part_to_disk(struct hd_struct *part)
 {
-	if (likely(part)) {
-		if (part->partno)
-			return dev_to_disk(part_to_dev(part)->parent);
-		else
-			return dev_to_disk(part_to_dev(part));
-	}
+	if (unlikely(!part))
 		return NULL;
+	return part->bdev->bd_disk;
 }
 
 static inline int disk_max_parts(struct gendisk *disk)