Add IP_FW_NAT64 to codes that ipfw_chk() can return.

It will be used by upcoming NAT64 changes. We use separate code
to avoid propogating EACCES error code to user level applications
when NAT64 consumes a packet.

Obtained from:	Yandex LLC
MFC after:	1 week
Sponsored by:	Yandex LLC

sys/netpfil/ipfw/ip_fw_pfil.c

@@ -313,6 +313,10 @@ ipfw_check_packet(struct mbuf **m0, struct ifnet *ifp, int dir,
 	case IP_FW_REASS:
 		goto again;		/* continue with packet */
 
+	case IP_FW_NAT64:
+		ret = PFIL_CONSUMED;
+		break;
+
 	default:
 		KASSERT(0, ("%s: unknown retval", __func__));
 	}

sys/netpfil/ipfw/ip_fw_private.h

@@ -61,6 +61,7 @@ enum {
 	IP_FW_NGTEE,
 	IP_FW_NAT,
 	IP_FW_REASS,
+	IP_FW_NAT64,
 };
 
 /*