Impact: rq->buffer usage cleanup

ide-atapi uses rq->buffer as private opaque value for internal special
requests.  rq->special isn't used for these cases (the only case where
rq->special is used is for ide-tape rw requests).  Use rq->special
instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>