sd-radv: send RA on update in caller side

As the caller may want to update multiple prefixes and friends.
Let's not automatically announce on update in library side.

Currently, only prefix is updated during running. Hence, this does not
change any effective behavior.

src/network/networkd-radv.c

@@ -761,9 +761,18 @@ int radv_add_prefix(
                 return r;
 
         r = sd_radv_add_prefix(link->radv, p);
-        if (r < 0 && r != -EEXIST)
+        if (r == -EEXIST)
+                return 0;
+        if (r < 0)
                 return r;
 
+        if (sd_radv_is_running(link->radv)) {
+                /* Announce updated prefixe now. */
+                r = sd_radv_send(link->radv);
+                if (r < 0)
+                        return r;
+        }
+
         return 0;
 }