io_uring: fix user_data field name in comment

io_uring_cqe's user_data field refers to `sqe->data`, but io_uring_sqe
does not have a data field. Fix the comment to say `sqe->user_data`.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://github.com/axboe/liburing/pull/1206
Link: https://lore.kernel.org/r/20240816181526.3642732-1-csander@purestorage.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>