igb/ixgbe: add IPV6_CSUM support to vlan_features

We were already exporting TSO6 to the vlan, but we weren't exporting the
checksum support for IPV6 which was causing warning messages to be
displayed when doing IPv6 TSO over a vlan.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>