fix the bug that IPv6 TCP connection fail

The code didn't create an new mld group when netif is up,
this will lead to the server can't find the matching netif
when receiving NS(neighbor solicitation) packet

src/core/ipv6/nd6.c

@@ -890,6 +890,15 @@ nd6_tmr(void)
 #endif
           /* @todo implement preferred and valid lifetimes. */
         } else if (netif->flags & NETIF_FLAG_UP) {
+#if ESP_LWIP
+#if LWIP_IPV6_MLD
+          if ((netif_ip6_addr_state(netif, i) & IP6_ADDR_TENTATIVE_COUNT_MASK) == 0) {
+            /* Join solicited node multicast group. */
+            ip6_addr_set_solicitednode(&multicast_address, netif_ip6_addr(netif, i)->addr[3]);
+            mld6_joingroup(netif_ip6_addr(netif, i), &multicast_address);
+          }
+#endif /* LWIP_IPV6_MLD */
+#endif
           /* Send a NS for this address. */
           nd6_send_ns(netif, netif_ip6_addr(netif, i), ND6_SEND_FLAG_MULTICAST_DEST);
           /* tentative: set next state by increasing by one */