liburing: fix issue with fast-pathed peek

There are actually two bugs in that commit:

- Assuming that __io_uring_peek_cqe() returns 0 with cqe always filled,
  like we do for the library functions. It can return 0 and not fill
  in the cqe, caller should check cqe. The internals do that, but not
  the new additions.

- We were checking for non-zero return, we should check for 0.

Link: https://github.com/axboe/liburing/issues/546
Fixes: 97a921456557 ("Make the fast path of CQE checking inline")
Signed-off-by: Jens Axboe <axboe@kernel.dk>