Minor clean up to xbd_queue_cb:

Minor clean up to xbd_queue_cb:
* nsegs must be at most BLKIF_MAX_SEGMENTS_PER_REQUEST (since we specify
  that limit to bus_dma_tag_create), so KASSERT that rather than silently
  adjusting the request.
* block_segs is now a synonym for nsegs, so garbage collect that variable.
* nsegs is never read during or after the while loop, so remove the dead
  decrement from the loop.

These were all left behind from the pre-r284296 support for a "segment
block" extension.