io_uring: limit local tw done

Instead of eagerly running all available local tw, limit the amount of
local tw done to the max of IO_LOCAL_TW_DEFAULT_MAX (20) or wait_nr. The
value of 20 is chosen as a reasonable heuristic to allow enough work
batching but also keep latency down.

Add a retry_llist that maintains a list of local tw that couldn't be
done in time. No synchronisation is needed since it is only modified
within the task context.

Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://lore.kernel.org/r/20241120221452.3762588-3-dw@davidwei.uk


Signed-off-by: Jens Axboe <axboe@kernel.dk>