Adjust MTU for IPv6 correctly

MTU for IPv6 is 20 bytes larger then IPv4. This fact was not taken into
account so IPv6 packets were larger then MTU resulting in fragmentation.

Solution is to substract correct IP header size.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>