cnetlink: properly handle ll neigh handable changes

With ae62117 ("cnetlink: pass on layer 2 neigh updates"), we make
sure to handle updates only on neighs we support, so we check both old
and new to be usable.

But this misses the transitions where a ll neigh becomes usable due to
updates or vice versa.

Fix this by treating those changes as a new or deleted ll neigh insted
of ignoring it alltogether.

Fixes: ae62117 ("cnetlink: pass on layer 2 neigh updates")
Signed-off-by: Jonas Gorski <[email protected]>

src/netlink/cnetlink.cc

@@ -1750,8 +1750,24 @@ void cnetlink::neigh_ll_created(rtnl_neigh *neigh) noexcept {
 
 void cnetlink::neigh_ll_updated(rtnl_neigh *old_neigh,
                                 rtnl_neigh *new_neigh) noexcept {
-  if (!check_ll_neigh(old_neigh) || !check_ll_neigh(new_neigh))
+  bool handle_old = check_ll_neigh(old_neigh);
+  bool handle_new = check_ll_neigh(new_neigh);
+
+  // nothing to do here
+  if (!handle_old && !handle_new)
+    return;
+
+  // we ignored the previous one, so this is like a new neigh
+  if (!handle_old) {
+    neigh_ll_created(new_neigh);
     return;
+  }
+
+  // we won't handle the neigh anymore
+  if (!handle_new) {
+    neigh_ll_deleted(old_neigh);
+    return;
+  }
 
   int old_ifindex = rtnl_neigh_get_ifindex(old_neigh);
   rtnl_link *old_base_link =