ixgbe: fix ipsec trailer length

Fix up the Tx trailer length calculation.  We can't believe the
trailer len from the xstate information because it was calculated
before the packet was put together and padding added.  This bit
of code finds the padding value in the trailer, adds it to the
authentication length, and saves it so later we can put it into
the Tx descriptor to tell the device where to stop the checksum
calculation.

Fixes: 59259470 ("ixgbe: process the Tx ipsec offload")
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>