firewall: Prevent spoofing our own RED IP address

firewall: Prevent spoofing our own RED IP address

There is no legitimate reason why traffic from our own IP address on RED
should ever appear incoming on that interface.

This prevents attackers from impersonating IPFire itself, and is only
cleared/reset if the RED interface is brought up. Therefore, an attacker
cannot bypass this by foring a dial-up or DHCP connection to break down.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>