io_uring: simplify task_work func

Since we don't really use req->task_work anymore, get rid of it together
with the nasty ->func aliasing between ->io_task_work and ->task_work,
and hide ->fallback_node inside of io_task_work.

Also, as task_work is gone now, replace the callback type from
task_work_func_t to a function taking io_kiocb to avoid casting and
simplify code.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>