ixgbe: Add support for UDP-encapsulated tx checksum offload

ixgbe: Add support for UDP-encapsulated tx checksum offload

Orabug: 21918732

By using GSO for UDP-encapsulated packets, all ixgbe devices can
be directed to generate checksums for the inner headers because
the outer UDP checksum can be zero. So point the machinery at the
inner headers and have the hardware generate the checksum.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit f467bc06022d4d37de459f9498ff4fbc7e9b0fca)
Signed-off-by: Brian Maly <brian.maly@oracle.com>