s390/qeth: add IPv6 TX checksum offload support

Check if a qeth device supports IPv6 TX checksum offload, and advertise
NETIF_F_IPV6_CSUM accordingly. Add support for setting the relevant bits
in IPv6 packet descriptors.

Currently this has only limited use (ie. UDP, or Jumbo Frames). For any
TCP traffic with a standard MSS, the TCP checksum gets calculated
as part of the linear GSO segmentation.

Signed-off-by: Kittipon Meesompop <kmeesomp@linux.vnet.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>