Neighbor cache update

Remove now only gargabe collected neighbours.

Change-Id: I013571fb024f1baca1cec6f3edbe91b7a4142666

Author: Juha Heiskanen

source/ipv6_stack/ipv6_routing_table.c

@@ -303,9 +303,15 @@ ipv6_neighbour_t *ipv6_neighbour_lookup_or_create(ipv6_neighbour_cache_t *cache,
 {
     uint_fast16_t count = 0;
     ipv6_neighbour_t *entry = NULL;
+    ipv6_neighbour_t *garbage_possible_entry = NULL;
 
     ns_list_foreach(ipv6_neighbour_t, cur, &cache->list) {
-        count++;
+
+        if (cur->type == IP_NEIGHBOUR_GARBAGE_COLLECTIBLE) {
+            garbage_possible_entry = cur;
+            count++;
+        }
+
         if (addr_ipv6_equal(cur->ip_address, address)) {
             if (cur != ns_list_get_first(&cache->list)) {
                 ns_list_remove(&cache->list, cur);
@@ -315,9 +321,9 @@ ipv6_neighbour_t *ipv6_neighbour_lookup_or_create(ipv6_neighbour_cache_t *cache,
         }
     }
 
-    if (count >= neighbour_cache_config.max_entries) {
-        entry = ns_list_get_last(&cache->list);
-        ipv6_neighbour_entry_remove(cache, entry);
+    if (count >= neighbour_cache_config.max_entries && garbage_possible_entry) {
+        //Remove Last storaged IP_NEIGHBOUR_GARBAGE_COLLECTIBLE type entry
+        ipv6_neighbour_entry_remove(cache, garbage_possible_entry);
     }
 
     // Allocate new - note we have a basic size, plus enough for the LL address,