io_uring: make io_fill_cqe_aux honour CQE32

Don't let io_fill_cqe_aux() post 16B cqes for CQE32 rings, neither the
kernel nor the userspace expect this to happen.

Fixes: 76c68fbf

 ("io_uring: enable CQE32")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/64fae669fae1b7083aa15d0cd807f692b0880b9a.1655287457.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>