loop: use BIO list management functions

Now that the bio list management stuff is generic, convert loop to use
bio lists instead of its own private bio list implementation.

Cc:  Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/loop.h

@@ -56,8 +56,7 @@ struct loop_device {
 	gfp_t		old_gfp_mask;
 
 	spinlock_t		lo_lock;
-	struct bio 		*lo_bio;
-	struct bio		*lo_biotail;
+	struct bio_list		lo_bio_list;
 	int			lo_state;
 	struct mutex		lo_ctl_mutex;
 	struct task_struct	*lo_thread;