i40e/i40evf: Update DESC_NEEDED value to reflect larger value

When compared to ixgbe and other previous Intel drivers the i40e and i40evf
drivers actually reserve 2 additional descriptors in maybe_stop_tx for
cache line alignment. We need to update DESC_NEEDED to reflect this as
otherwise we are more likely to return TX_BUSY which will cause issues with
things like xmit_more.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/i40e/i40e_txrx.h

@@ -279,7 +279,7 @@ static inline unsigned int i40e_txd_use_count(unsigned int size)
 }
 
 /* Tx Descriptors needed, worst case */
-#define DESC_NEEDED (MAX_SKB_FRAGS + 4)
+#define DESC_NEEDED (MAX_SKB_FRAGS + 6)
 #define I40E_MIN_DESC_PENDING	4
 
 #define I40E_TX_FLAGS_HW_VLAN		BIT(1)

drivers/net/ethernet/intel/i40evf/i40e_txrx.h

@@ -260,7 +260,7 @@ static inline unsigned int i40e_txd_use_count(unsigned int size)
 }
 
 /* Tx Descriptors needed, worst case */
-#define DESC_NEEDED (MAX_SKB_FRAGS + 4)
+#define DESC_NEEDED (MAX_SKB_FRAGS + 6)
 #define I40E_MIN_DESC_PENDING	4
 
 #define I40E_TX_FLAGS_HW_VLAN		BIT(1)