o Always use list_del_init() on request queuelist, this allows us to
  sanity check the integrity of the request on insertion and removal.
  So we can complain loudly instead of silently corrupting memory.

o Add references to requests. This is cheap, since we dont have to use
  an atomic variable for it (all puts are inside queue lock). We've had
  a bug in IDE for years where we want to inspect request state after
  io completion, but this is not possible to do race free right now.
  REQ_BLOCK_PC and sgio will need this too, for checking io residual
  etc. This is not just a theoretical race, I've seen it happen.