io: do not use IO_FORGOTTEN_FUTURES_SKIP_CQE on linux <= 5.17

Apparantly my assumption that io_uring ignores unknown cqe flags is
wrong. And io_uring < Linux 5.17 completes forgotten futures with
-EINVAL.

Fixes: e140759d
49 jobs for fix-forgotten-futures-on-linux-lt-5.17 in 18 minutes and 44 seconds (queued for 2 seconds)