io_uring: Add to traces the req pointer when available

The req pointer uniquely identify a specific request.
Having it in traces can provide valuable insights that is not possible
to have if the calling process is reusing the same user_data value.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>