io_uring/zctx: separate notification user_data

io_uring/zctx: separate notification user_data

People previously asked for the notification CQE to have a different
user_data value from the main request completion. It's useful to
separate buffer and request handling logic and avoid separately
refcounting the request.

Let the user pass the notification user_data in sqe->addr3. If zero,
it'll inherit sqe->user_data as before. It doesn't change the rules for
when the user can expect a notification CQE, and it should still check
the IORING_CQE_F_MORE flag.

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