Don't discard packets with 'Destination Unreachable' at the beginning
of ip_forward(), if the IPSEC is compiled in. It is possible that there
is an SPD that this packets will go through, even if there is no matching
route. If not, ICMP will be sent anyway, after ip_output().

This is somewhat similar in purpose to r191621, except that one was
for the packets sent from the host, while this one is for packets
being forwarded by the host.

Reviewed by: bz@
Sponsored by: Wheel Sp. z o.o. (http://www.wheel.pl)