[$] Lockless MPSC FIFO queues for io_uring

Processes that use io_uringtend to keep a lot of balls in the air; being able to have many operationsunderway at any given time is part of the point of that API in the firstplace. The io_uring subsystem must, as a result, keep track of a lot oftasks that have to be performed at the right time. In current kernels,io_uring uses a standard kernel linked-list primitive to track those workitems. As of the 7.2 kernel release, though, io_uring will, instead, use anew lockless, multi-producer, single-consumer (MPSC) queue, resulting insome notable performance gains. Lockless algorithms tend to be tricky, butthe one used here is relatively approachable and shows how these algorithmscan work.