rua: validate correct RUA ctx state per RUA message type

It helps in a pretty complex situation seen in the field.
A third-party MSC releases SCCP in one fell swoop, not waiting for the
Iu Release Complete to come back from RAN as the specs would suggest.

The result is this odd sequence, where late rx of RUA Disconnect
actually causes a new SCCP connection to be established and torn down
again:

    RAN                 HNBGW                MSC
     |--active-RUA-ctx----|--active-sccp------|
     |                    |<--IU-Release-Cmd--|
     |<--IU-Release-Cmd---|                   |
     |                    |<--SCCP-RLSD-------| (too soon)
     |...<-RUA-Disconnect-|                   x (the consequence)
     |                    x
     |--RUA-Disconnect--->|                     (IU Release Complete)
     x              <create-new-ctx>
                          |-SCCP-CR---------->|
                          |-IU-Release-Compl->|
                          |<--CREF------------|
                          x                   x

This patch is a relatively simple practical improvement of above
situation that is logically obvious:

Validate the correct message type for creating a new RUA-to-SCCP
context: RUA Connect.

That means the IU Release Complete is ignored:

    RAN                 HNBGW                MSC
     |--active-RUA-ctx----|--active-sccp------|
     |                    |<--IU-Release-Cmd--|
     |<--IU-Release-Cmd---|                   |
     |                    |<--SCCP-RLSD-------| (too soon)
     |...<-RUA-Disconnect-|                   x (the consequence)
     |                    x
     |--RUA-Disconnect--->|                     (IU Release Complete)
     x                 <error>
                          x

Related: SYS#6602
Change-Id: Ie359fcada98fb19f56015cf462e6d8c039f5fce5
---
M src/osmo-hnbgw/hnbgw_rua.c
1 file changed, 82 insertions(+), 14 deletions(-)