In the event of an icmp packet, take only the ports instead of trying to
grab the full header.

In the event of an inverse packet, use the source address and port.
Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>