ixgbevf: Handle extended IPv6 headers in Tx path

Check for and handle IPv6 extended headers so that Tx checksum
offload can be done. Also use skb_checksum_help for unexpected
cases. Thanks to Tom Herbert for noticing these problems. Thanks
to Alexander Duyck for seeing how to coalesce the error handling
into one location.

Reported-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Darin Miller <darin.j.miller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>