Forward AMF UE NGAP ID to RRC on DL NAS Message

In the Initial Context Setup message, the NGAP forwards the AMF UE ID,
but not in the DL NAS Message, although 38.413 says, in $8.3.1.2 either

> The NG-RAN node shall use the AMF UE NGAP ID IE and RAN UE NGAP ID IE
> received in the INITIAL CONTEXT SETUP REQUEST message as identification
> of the logical connection

or in $8.6.2.2

> The NG-RAN node shall use the AMF UE NGAP ID IE and RAN UE NGAP ID IE
> received in the DOWNLINK NAS TRANSPORT message as identification of the
> logical connection

So forward in both cases for harmonization.

openair2/RRC/NR/rrc_gNB_NGAP.c

@@ -717,6 +717,7 @@ int rrc_gNB_process_NGAP_DOWNLINK_NAS(MessageDef *msg_p, instance_t instance, mu
   }
 
   gNB_RRC_UE_t *UE = &ue_context_p->ue_context;
+  UE->amf_ue_ngap_id = req->amf_ue_ngap_id;
   UE->nas_pdu = req->nas_pdu;
   rrc_forward_ue_nas_message(rrc, UE);
   return 0;