It allows for much better control on the lifecycle of the session, and
already shows some missing tear down paths in case of errors.
It also clarifies the existence of "sess" pointer in several paths.
The code also becomes clearer overall, since all the transitions and
logic to send next messages are put together.
Tear down of the session will be integrated into gsm-sm in a follow-up
patch.
The 5gc session setup is only partially moved to gsm-sm, and left as an
exercise for users wishin to improve 5gc support.
* [SMF] Gn: Avoid assert crash if no PDP resources available
* [SMF] Gn: Rearrange IE handling order in CreatePDPContextRequest
Let's handle the GTPC remote addr + TEID first, since those should be
used in the CreatePDPContextResponse ideally if available.
Let's then handle parsing of all IEs not related to bearers/UserPlane,
then those missing, and finally do all the IP resource allocation.