Interpret sl3a_uport == 0 in SVP_R_VL3_ACK to indicate the VL3 IP is a
next-hop router.

@@ -275,11 +275,12 @@
         uint16_t                svp_port;       /* svp_lock */
         uint16_t                svp_uport;      /* svp_lock */
         uint32_t                svp_dcid;       /* svp_lock (but write-once?) */
         boolean_t               svp_huip;       /* svp_lock */
         struct in6_addr         svp_uip;        /* svp_lock */
-        struct ether_addr       svp_router_mac; /* svp_lock (but write-once?) */
+        /* NOTE: lower-3 bytes are 0s. */
+        uint8_t         svp_router_oui[6];      /* svp_lock (but write-once?) */
 };
 
 extern bunyan_logger_t *svp_bunyan;
 
 extern int svp_remote_find(char *, uint16_t, struct in6_addr *,