IORING_OP_MSG_RING should have a `io_uring_register_msg_ring`

[davidzeng0]

This would make it simple for thread pools to send completed tasks to a ring without creating a ring of its own. It would also avoid the need to set up futexes/event fds and polling them on the ring.

[davidzeng0]

Interestingly the eventfd method has 3x less latency than MSG_RING (5us vs 15us) despite how simple the code for that op is. Maybe it's the submission path through io_uring_enter that adds unnecessary overhead. Could still be a useful feature though