Add version check in Geneve dissector.

We should warn when decoding Geneve packets with an unknown
version number.

Change-Id: Id40b756c3bb0320b69fbd8ee98830a2b05834a48
Reviewed-on: https://code.wireshark.org/review/5420
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>