block, blk-sysfs: Use the variable directly instead of a function call

In the function blk_register_queue(), var _dev_ is already assigned by
disk_to_dev().So use it directly instead of calling disk_to_dev() again.
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>

Modified by me to delete an empty line in the same function while
in there anyway.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-sysfs.c

@@ -498,7 +498,6 @@ int blk_register_queue(struct gendisk *disk)
 {
 	int ret;
 	struct device *dev = disk_to_dev(disk);
-
 	struct request_queue *q = disk->queue;
 
 	if (WARN_ON(!q))
@@ -521,7 +520,7 @@ int blk_register_queue(struct gendisk *disk)
 	if (ret) {
 		kobject_uevent(&q->kobj, KOBJ_REMOVE);
 		kobject_del(&q->kobj);
-		blk_trace_remove_sysfs(disk_to_dev(disk));
+		blk_trace_remove_sysfs(dev);
 		kobject_put(&dev->kobj);
 		return ret;
 	}