Construct the concatenated icmp packet for error replies after setting the checksum, not before.
Construct the concatenated icmp packet for error replies after setting the checksum, not before.

Signed-off-by: Mindy Preston <643594450af9759cecd6ce8d2d2440748cdf4e27@docker.com>

djs55/vpnkit,djs55/vpnkit,samoht/vpnkit,djs55/vpnkit