block/loop: remove unused field

nobody uses the list.
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.h

@@ -67,7 +67,6 @@ struct loop_device {
 struct loop_cmd {
 	struct kthread_work work;
 	struct request *rq;
-	struct list_head list;
 	union {
 		bool use_aio; /* use AIO interface to handle I/O */
 		atomic_t ref; /* only for aio */