i40e/i40evf: Limit TSO to 7 descriptors for payload instead of 8 per packet

This patch addresses a bug introduced based on my interpretation of the
XL710 datasheet.  Specifically section 8.4.1 states that "A single transmit
packet may span up to 8 buffers (up to 8 data descriptors per packet
including both the header and payload buffers)."  It then later goes on to
say that each segment for a TSO obeys the previous rule, however it then
refers to TSO header and the segment payload buffers.

I believe the actual limit for fragments with TSO and a skbuff that has
payload data in the header portion of the buffer is actually only 7
fragments as the skb->data portion counts as 2 buffers, one for the TSO
header, and one for a segment payload buffer.

Fixes: 2d37490b ("i40e/i40evf: Rewrite logic for 8 descriptor per packet check")
Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>