virtio_blk - Clean up some unneeded functionalities.

virtio_blk - Clean up some unneeded functionalities.

* Get rid of remaining code for handling the B_ORDERED flag.
  B_ORDERED doesn't even exist anymore in DragonFly.

* Get rid of sc->vtblk_req_ready request list. We can just use
  bioqdisksort() to add a request - that we failed to enqueue ino the
  virtqueue - back into the bioq.

* Rename req->vbr_bp field to req->vbr_bio.

* Switch vtblk_req_free list from TAILQ to SLIST. A small optimization,
  none of the additional features of the TAILQ are needed here.