blk-core: use BIO list management functions

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

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