ixgbe: Add support for UDP-encapsulated tx checksum offload

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>