GMM: permit VLR_ANSWERED event in attach FSM

This fixes following error:
DMM gprs_gmm.c:1126
    GMM_ATTACH_REQ_FSM(gb_gmm_req)[0x5589e78dded0]{WaitAttachComplete}:
	Event VLR answered not permitted

There seems to be a race condition in FSM when MS establishes MM context
which isn't immediately followed up by PDP context (for example when no
APN is configured in MS).

This does not affect actual functionality because in this case MS won't
be able to use GPRS anyway but it's still nice to get our FSM fixed even
in this corner case.

Change-Id: I14d234632224e20faf865d2273c83cfff31abf61

src/gprs/gprs_gmm_attach.c

@@ -195,6 +195,12 @@ static void st_accept(struct osmo_fsm_inst *fi, uint32_t event, void *data)
 		extract_subscr_hlr(ctx);
 		osmo_fsm_inst_state_chg(fi, ST_INIT, 0, 0);
 		break;
+	case E_VLR_ANSWERED:
+		extract_subscr_msisdn(ctx);
+		extract_subscr_hlr(ctx);
+		LOGMMCTXP(LOGL_NOTICE, ctx,
+			  "Unusual event: if MS got no data connection, check that it has APN configured.\n");
+		break;
 	}
 }
 
@@ -295,7 +301,7 @@ static struct osmo_fsm_state gmm_attach_req_fsm_states[] = {
 		.action = st_iu_security_cmd,
 	},
 	[ST_ACCEPT] = {
-		.in_event_mask = X(E_ATTACH_COMPLETE_RECV),
+		.in_event_mask = X(E_ATTACH_COMPLETE_RECV) | X(E_VLR_ANSWERED),
 		.out_state_mask = X(ST_INIT) | X(ST_REJECT),
 		.name = "WaitAttachComplete",
 		.onenter = st_accept_on_enter,