[MME] Fixed Error Indication (#3302)

We've encountered an issue where Downlink Data Notification Acks are not sent
in the following situations.

1. SGW-U received Error Indication
2. SGW-U sends PFCP Report Request to SGW-C
3. SGW-C sends PFCP Report Response to SGW-U
4. SGW-C sends Downlink Data Notification to MME
(MME Connected with eNB)
5. MME sends UEContextReleaseCommand to the eNB
6. eNB sends UEContextReleaseComplete to the MME
7. MME sends S1-Paging to the eNB
8. eNB sends Service-Request to the MME
9. MME sends InitialContextSetupRequest to the eNB
10. eNB sends InitialContextSetupResponse to the MME

Here, MME needs to send Downlink Data Notification Acknowledge.

So, we've fixed it

src/mme/mme-s11-handler.c

@@ -1645,6 +1645,8 @@ void mme_s11_handle_downlink_data_notification(
             enb_ue_t *enb_ue = enb_ue_find_by_id(mme_ue->enb_ue_id);
             ogs_assert(enb_ue);
 
+            MME_STORE_PAGING_INFO(mme_ue,
+                MME_PAGING_TYPE_DOWNLINK_DATA_NOTIFICATION, bearer->id);
             r = s1ap_send_ue_context_release_command(enb_ue,
                     S1AP_Cause_PR_nas, S1AP_CauseNas_normal_release,
                     S1AP_UE_CTX_REL_S1_PAGING, 0);