io_uring: cleanup handle_tw_list() calling convention

Now that we don't loop around task_work anymore, there's no point in
maintaining the ring and locked state outside of handle_tw_list(). Get
rid of passing in those pointers (and pointers to pointers) and just do
the management internally in handle_tw_list().

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