io_uring/waitid: setup async data in the prep handler

This is the idiomatic way that opcodes should setup their async data,
so that it's always valid inside ->issue() without issue needing to
do that.

Fixes: f31ecf67 ("io_uring: add IORING_OP_WAITID support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>