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>