firewall: fix forwarding local subnet traffic

firewall: fix forwarding local subnet traffic

Packets which are merely forwarded by the router and which are neither
involved in any DNAT/SNAT nor originate locally, are considered INVALID
from a conntrack point of view, causing them to get dropped in the
zone_*_dest_ACCEPT chains, since those only allow stream with state NEW
or UNTRACKED.

Remove the ctstate restriction on dest accept chains to properly pass-
through unrelated 3rd party traffic.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>