sgsn: Store subscribed QoS and attempt to use it

sgsn_create_pdp_ctx should use the subscribed QoS. When selecting
the PDP context we inject the QoS to be used into the TLV structure
and use it during the request. Assume a "qos-Subscribed" structure
only with three bytes and prepend the Allocation/Retention policy
to the request.
diff --git a/openbsc/include/openbsc/gprs_sgsn.h b/openbsc/include/openbsc/gprs_sgsn.h
index baa2d78..7a429cd 100644
--- a/openbsc/include/openbsc/gprs_sgsn.h
+++ b/openbsc/include/openbsc/gprs_sgsn.h
@@ -294,6 +294,8 @@
 	unsigned int		context_id;
 	uint16_t		pdp_type;
 	char			apn_str[GSM_APN_LENGTH];
+	uint8_t			qos_subscribed[20];
+	size_t			qos_subscribed_len;
 };
 
 struct sgsn_subscriber_data {