block, cfq: move cfqd->icq_list to request_queue and add request->elv.icq

Most of icq management is about to be moved out of cfq into blk-ioc.
This patch prepares for it.

* Move cfqd->icq_list to request_queue->icq_list

* Make request explicitly point to icq instead of through elevator
  private data.  ->elevator_private[3] is replaced with sub struct elv
  which contains icq pointer and priv[2].  cfq is updated accordingly.

* Meaningless clearing of ->elevator_private[0] removed from
  elv_set_request().  At that point in code, the field was guaranteed
  to be %NULL anyway.

This patch doesn't introduce any functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>