Changes

fixup: sgsn: fix unused param in f_routing_area_update()

In 1ee1edd2 I changed f_routing_area_update() to actually use the
given RAI as Old RAI in the Routing Area Update Request.  Not only
this broke the testcase scenario (Old RAI shall remain unchanged!),
but also started triggering a use-after-free bug in osmo-sgsn.

Passing 'ran_index := 1' is enough for the second Routing Area Update
Request to show up with a different RAI (at BSSGP level), however the
Old RAI IE shall obviously indicate the *old* RAI, not the new one.

A follow-up commit will add a separate testcase to reproduce the
use-after-free problem in osmo-sgsn.

Change-Id: Ib16985cb08834a238ca4f7a747c43097f430ed6f
Fixes: 1ee1edd2 "sgsn: fix unused param in f_routing_area_update()"
Related: OS#6439