ixgbe: Handle extended IPv6 headers in Tx path

ixgbe: Handle extended IPv6 headers in Tx path

Orabug: 23177316

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 recognizing problems with the first version
of this patch and recognizing how to coalesce error conditions
into a single location.

Reported-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 36a92d7190e68e9387347695fe4625eb2c9e7e1c)
Signed-off-by: Brian Maly <brian.maly@oracle.com>