From Petr Vandrovec

   we use nbd for our diskless systems, and it looks to me like that
it has some serious problems in 2.5.x... Can you apply this patch
and forward it to Linus?

There were:
* Missing disk's queue initialization
* Driver should use list_del_init: put_request now verifies
  that req->queuelist is empty, and list_del was incompatible
  with this.
* I converted nbd_end_request back to end_that_request_{first,last}
  as I saw no reason why driver should do it itself... and
  blk_put_request has no place under queue_lock, so apparently when
  semantic changed nobody went through drivers...