bfq-iosched: ensure to clear bic/bfqq pointers when preparing request

Even if we don't have an IO context attached to a request, we still
need to clear the priv[0..1] pointers, as they could be pointing
to previously used bic/bfqq structures. If we don't do so, we'll
either corrupt memory on dispatching a request, or cause an
imbalance in counters.

Inspired by a fix from Kees.

Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Fixes: aee69d78 ("block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler")
Signed-off-by: Jens Axboe <axboe@kernel.dk>