Conflux linked cell not counted as non-dropped valid data

The vanguards addon is counting the conflux linked cell as a dropped cell because we forgot to count it as valid data if it was successfully handled: https://github.com/mikeperry-tor/vanguards/issues/95

Fix is simple, we just need to add a call to circuit_read_valid_data() after handling linked. We don't need it for the other conflux commands because they don't arrive at origin.