io_uring: pass in struct io_big_cqe to io_alloc_ocqe()

Rather than pass extra1/extra2 separately, just pass in the (now) named
io_big_cqe struct instead. The callers that don't use/support CQE32 will
now just pass a single NULL, rather than two seperate mystery zero
values.

Move the clearing of the big_cqe elements into io_alloc_ocqe() as well,
so it can get moved out of the generic code.

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