network: several fixlets for NDisc (#42218)

Unfortunately, previously the path to test-ndisc-send has been wrong, so
some test cases have not been checked in our mkosi CIs. And two test
cases have been broken.

The test case `test_ndisc_redirect` was not updated when the logic in
networkd was changed by 9142bd5a8e. The
change itself should be OK. So, the test case is updated.

The test case `test_ndisc_mtu` was broken when the commit
32417c1723 is merged. The commit is not
correct, as we cannot set IPv6 MTU larger than interface MTU. So, the
offending commit is reverted.

src/network/networkd-sysctl.c

@@ -545,11 +545,11 @@ int link_set_ipv6_mtu(Link *link, int log_level) {
         if (mtu == 0)
                 return 0;
 
-        if (mtu > link->max_mtu) {
+        if (mtu > link->mtu) {
                 log_link_full(link, log_level,
                               "Reducing requested IPv6 MTU %"PRIu32" to the interface's maximum MTU %"PRIu32".",
-                              mtu, link->max_mtu);
-                mtu = link->max_mtu;
+                              mtu, link->mtu);
+                mtu = link->mtu;
         }
 
         r = sysctl_write_ip_property_uint32(AF_INET6, link->ifname, "mtu", mtu, manager_get_sysctl_shadow(link->manager));