Thread-identity switcheroo for io_uring

If like me you read this and wondered if it might be simpler to just swap stacks between task_structs rather than leaving the stacks in place and swapping everything else, it came up in the discussion on the mailing list: https://lwn.net/ml/all/74c87cd3-8ac1-46da-a1b9-9149d46b4788@kernel.dk/