ipvs: Maintain all DSCP and ECN bits for ipv6 tun forwarding

Previously, only the four high bits of the tclass were maintained in the
ipv6 case.  This matches the behavior of ipv4, though whether or not we
should reflect ECN bits may be up for debate.

Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>