queue: add opt-in requeue helper for SQ_REWIND

[carrerasdarren-cell]

IORING_SETUP_SQ_REWIND clears liburing's local SQ tail before entering the kernel. If a submission is short, the caller may either abandon the unconsumed entries or choose to retry them. Retrying requires relocating those entries to the beginning of the SQ.

This adds io_uring_sq_requeue() as an explicit opt-in helper. The application records io_uring_sq_ready() before submission, then passes that count and the submission return value to the helper before acquiring any new SQEs.

Submission-and-wait paths also stop internally resubmitting an unrelocated SQ_REWIND remainder. Without that guard, a short timed submission can submit already-consumed slots again.

The regression covers:

• the default abandon policy and explicit requeue policy;
• ordinary and fixed 128-byte SQEs;
• partial and fully failed submissions;
• timed submission with duplicate-completion detection;
• a pure wait with locally pending work; and
• the regular circular-ring behavior as a control.

Validation:

• Linux 7.0.0-28-generic
• normal GCC build and FFI symbol check
• ASan/UBSan build
• focused regression plus nine adjacent submission/SQ tests in both builds

[krisman]

I don't think this should be done by default on a short submit. We might not want to requeue, in which case relocating the requests would be a waste of time. Perhaps the relocation could be exposed as a helper in io_uring and let userspace call it if they want.

[carrerasdarren-cell]

Agreed. I updated the patch so a short SQ_REWIND submit is abandoned by default and added io_uring_sq_requeue() as an explicit opt-in helper.

I also stopped the submission-and-wait loop from internally submitting an unrelocated remainder, since that can resubmit the already-consumed prefix. The revised regression covers both policies, partial and failed submits, fixed 128-byte SQEs, and timed-submit duplicate detection. Normal and ASan/UBSan builds plus ten focused tests pass on Linux 7.0.

I force-pushed the revised single commit. I am happy to adjust the helper name or signature if you prefer a different API shape.