io_uring/waitid: use io_is_compat()

Use io_is_compat() for consistency.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://lore.kernel.org/r/28c5b5f1f1bf7f4d18869dafe6e4147ce1bbf0f5.1740400452.git.asml.silence@gmail.com
Link: https://lore.kernel.org/r/20250224172337.2009871-1-csander@purestorage.com


[axboe: fold in improvement from Caleb, see link]
Signed-off-by: Jens Axboe <axboe@kernel.dk>