Re-introduce kernel GTP support in OsmoGGSN
Due to the different way how GGSN instances / APNs are handled in OsmoGGSN vs. OpenGGSN, the kernel GTP support had to be temporarily disabled.
We need to re-introduce it.
- % Done changed from 30 to 80
A series of patches has been posted at https://gerrit.osmocom.org/#/q/status:open+project:osmo-ggsn+branch:master+topic:kernel-gtp and will be in review. Patches have been manually verified with 4.9, 4.12 and a 4.15-rc1 kernel.