io_uring: Fix danger of completion getting reused before being read

Hi,

I privately got a report about a bug in method_io_uring.c:

We are calling io_uring_cqe_seen(..., cqe) before reading cqe->res. That
allows the completion to be reused, which in turn could lead to cqe->res being
overwritten. The window for that is very narrow and the likelihood of it
happening is very low, as we should never actually utilize all CQEs, but the
consequences would be bad.

I plan to push the fix soon-ish.

Greetings,

Andres Freund