Changes

aaa_diameter_swx: Avoid answering RTR with Success if already deregistering

When receiving an SWx RTR on our osmo-epdg (epdg+AAA), we initiate the
deregistration against peer nodes on other interfaces and answer with SWx RTA.

If a subsequent RTR for the subscriber arrives on the interface while
the deregistration is in progress, we'd answer with RTA Result-Code 2001
SUCCESS. Let's instead answer already as if we didn't know about the
subscriber, since it will end up being freed in a short span of time and
nothing can be done about it.

Related: OS#6796
Change-Id: I0ed13e9503ec380ce4331c662ca3eb966ccbda92