interface-ip: fine tune IPv6 mtu warning

Failing to set the IPv6 mtu is only a real issue if the mtu to be set is
smaller than the current mtu as in that case it would break IPv6 path mtu;
adapt the logic to generate the warning accordingly

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

interface-ip.c

@@ -982,7 +982,7 @@ interface_set_prefix_address(struct device_prefix_assignment *assignment,
 			int mtu_old = system_update_ipv6_mtu(l3_downlink, 0);
 
 			if (mtu > 0 && mtu_old != mtu) {
-				if (system_update_ipv6_mtu(l3_downlink, mtu) < 0)
+				if (system_update_ipv6_mtu(l3_downlink, mtu) < 0 && mtu < mtu_old)
 					netifd_log_message(L_WARNING, "Failed to set IPv6 mtu to %d "
 							"on interface '%s'\n", mtu, iface->name);
 			}