trans_lcls_compose(): Set PLMN fron cell currently in use

The MS in general provides the Selected PLMN ID (IE) in the Complete
Layer 3 Information message. osmo-msc handles that message in
msc_a_ran_dec_from_msc_i() and stores the information of the PLMN in
msc_a->via_cell. If no PLMN information is provided in the message, then
at that same place the PLMN configured in the VTY is taken as an implicit
default.

This patch changes trans_lcls_compose() to use the PLMN stored in
msc_a->via_cell instead of the VTY configured one, meaning the PLMN
provided by the MS (through the RAN in use) is used if available
(otherwise the VTY-configure one is still used, as before).

With this patch the PLMN VTY config option use is relegated to a single
point of use in msc_a_ran_dec_from_msc_i() where the Complete Layer 3
Information is used. As a result, it becomes clear now that the VTY
config is only applied in the scenario where no PLMN is provided at that
time.

Related: SYS#6360
Change-Id: Ibad0005a1d7cef64dd8fefa3e554ba99a06c3666
---
M src/libmsc/transaction.c
1 file changed, 29 insertions(+), 1 deletion(-)