io_uring: cqe init hardening

io_kiocb::cqe stores the completion info which we'll memcpy to
userspace, and we rely on callbacks and other later steps to populate
it with right values. We have never had problems with that, but it would
still be safer to zero it on allocation.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b16a3b64dde678686460d3c3792c3ba6d3d1bc7a.1692916914.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>