network: delay dropping addresses or so on reloading .network files

When a .network file is updated but its change is not so big, it is not
necessary to first drop all configs and then reassign later again.
This slightly optimize such situation. First foreignize all configs, and
then drop later when it is not requested by the updated .network file.

src/network/networkd-link.c

@@ -1051,6 +1051,17 @@ static int link_drop_config(Link *link) {
         return r;
 }
 
+static void link_foreignize_config(Link *link) {
+        assert(link);
+        assert(link->manager);
+
+        link_foreignize_routes(link);
+        link_foreignize_nexthops(link);
+        link_foreignize_addresses(link);
+        link_foreignize_neighbors(link);
+        link_foreignize_routing_policy_rules(link);
+}
+
 static int link_configure(Link *link) {
         int r;
 
@@ -1249,9 +1260,16 @@ static int link_reconfigure_impl(Link *link, bool force) {
 
         link_drop_requests(link);
 
-        r = link_drop_config(link);
-        if (r < 0)
-                return r;
+        if (network && !force)
+                /* When a new/updated .network file is assigned, first make all configs (addresses,
+                 * routes, and so on) foreign, and then drop unnecessary configs later by
+                 * link_drop_foreign_config() in link_configure(). */
+                link_foreignize_config(link);
+        else {
+                r = link_drop_config(link);
+                if (r < 0)
+                        return r;
+        }
 
         link_free_carrier_maps(link);
         link_free_engines(link);