IORing: support SQPOLL

I would like to use iouring with SQPOLL to avoid unnecessary syscalls.

I found this comment in the code and wonder if there are any plans of supporting it...
```
// Ring always needs enter right now;
// TODO: support SQPOLL here
while true {
let ret = io_uring_enter(ringDescriptor, numEvents, minCompletions, flags, nil)

```

Start by locating the ring operation containing the shown io_uring_enter loop and read how its flags and completion parameters are assembled. Investigate the SQPOLL behavior and determine what changes are needed so the ring can avoid unnecessary enters; done means SQPOLL rings operate correctly without breaking existing ring behavior, with tests covering the supported path.

Written by the indexing model from the issue text.