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.

(backported from master 03f95ddaa1d223e1e93788a307dc1b36d86b22b5)

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