autoip: Choose next address after rate limit

AutoIP now selects a new address after rate limit timeout,
AutoIP tries a new address by incrementing the tried_llipaddr counter
in the ACD_DECLINE case of the callback.

In lwIP pre-2.2.0, address conflict detection was handled within autoip.c, and
the incrementing happened in autoip_restart() (line 150). When ACD was
extracted into a separate module in 2.2.0, this increment was missing for the
rate-limiting path.

Without this change, devices continuously retry the same IP address after rate
limiting, causing them to fail Bonjour Conformance Tests.

Author: david-cermak

src/core/ipv4/autoip.c

@@ -223,9 +223,10 @@ autoip_conflict_callback(struct netif *netif, acd_callback_enum_t state)
       autoip_restart(netif);
       break;
     case ACD_DECLINE:
-      /* "delete" conflicting address so a new one will be selected in
-       * autoip_start() */
+      /* "delete" conflicting address and increment tried addr so a new one
+       * will be selected in autoip_start() */
       ip4_addr_set_any(&autoip->llipaddr);
+      autoip->tried_llipaddr++;
       autoip_stop(netif);
       break;
       default: