Drop to-be-forwarded-nowhere packets on wans

Dropping packets with no clear forward destination is nicer than
rejecting them. Especially when some providers punish users for spoofing
caused by their noisy infra.

Fixes: openwrt/openwrt#13340

root/etc/config/firewall

@@ -19,7 +19,7 @@ config zone
 	list   network		'wan6'
 	option input		REJECT
 	option output		ACCEPT
-	option forward		REJECT
+	option forward		DROP
 	option masq		1
 	option mtu_fix		1