io_uring: make io_alloc_ocqe() take a struct io_cqe pointer

The number of arguments to io_alloc_ocqe() is a bit unwieldy. Make it
take a struct io_cqe pointer rather than three separate CQE args. One
path already has that readily available, add an io_init_cqe() helper for
the remaining two.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>