io_uring: fix nvme's 32b cqes on mixed cq

The nvme uring_cmd only uses 32b CQEs. If the ring uses a mixed CQ, then
we need to make sure we flag the completion as a 32b CQE.

On the other hand, if nvme uring_cmd was using a dedicated 32b CQE, the
posting was missing the extra memcpy because it only applied to bit CQEs
on a mixed CQ.

Fixes: e26dca67 ("io_uring: add support for IORING_SETUP_CQE_MIXED")
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>