network/ndisc: also drop SLAAC addresses and routes on reconfiguring

Especially, this is important for routes, as link_drop_managed_routes()
removes only static routes, rather than non-foreign routes.

src/network/networkd-ndisc.c

@@ -1640,7 +1640,8 @@ int ndisc_stop(Link *link) {
 void ndisc_flush(Link *link) {
         assert(link);
 
-        /* Remove all RDNSS, DNSSL, and Captive Portal entries, without exception. */
+        /* Remove all addresses, routes, RDNSS, DNSSL, and Captive Portal entries, without exception. */
+        (void) ndisc_drop_outdated(link, /* timestamp_usec = */ USEC_INFINITY);
 
         link->ndisc_rdnss = set_free(link->ndisc_rdnss);
         link->ndisc_dnssl = set_free(link->ndisc_dnssl);