network: drop old ndisc configurations after new ones are configured

Previously, `ndisc_remove_old_one()` checked `ndisc_{addresses,routes}_configured`
flags, but they are not unset when all addresses or routes are already
assigned.
After the request queue is implemented, the address or route requests
are not processed within the same event of ndisc handler is called, but
will processed later when they are ready. So, calling `ndisc_remove_old()`
in the event of ndisc handler will remove all addresses and routes
previously assigned even they are requested to be updated.

This makes `ndisc_remove_old()` do nothing when there exist some
requests to configure addresses and routes, thus previously assigned
addresses and routes are kept until all requests are processed.

Fixes systemd#20050.

src/network/networkd-ndisc.c

@@ -128,9 +128,6 @@ static int ndisc_remove_old_one(Link *link, const struct in6_addr *router, bool
         if (!force) {
                 bool set_callback = false;
 
-                if (!link->ndisc_addresses_configured || !link->ndisc_routes_configured)
-                        return 0;
-
                 SET_FOREACH(na, link->ndisc_addresses)
                         if (!na->marked && in6_addr_equal(&na->router, router)) {
                                 set_callback = true;
@@ -212,6 +209,10 @@ static int ndisc_remove_old(Link *link) {
 
         assert(link);
 
+        if (link->ndisc_addresses_messages > 0 ||
+            link->ndisc_routes_messages > 0)
+                return 0;
+
         routers = set_new(&in6_addr_hash_ops);
         if (!routers)
                 return -ENOMEM;