i40e: Limit TX descriptor count in cases where frag size is greater than 16K

The i40e driver was incorrectly assuming that we would always be pulling
no more than 1 descriptor from each fragment.  It is in fact possible for
us to end up with the case where 2 descriptors worth of data may be pulled
when a frame is larger than one of the pieces generated when aligning the
payload to either 4K or pieces smaller than 16K.

To adjust for this we just need to make certain to test all the way to the
end of the fragments as it is possible for us to span 2 descriptors in the
block before us so we need to guarantee that even the last 6 descriptors
have enough data to fill a full frame.

Change-ID: Ic2ecb4d6b745f447d334e66c14002152f50e2f99
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>

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

@@ -2621,9 +2621,7 @@ bool __i40e_chk_linearize(struct sk_buff *skb)
 		return false;
 
 	/* We need to walk through the list and validate that each group
-	 * of 6 fragments totals at least gso_size.  However we don't need
-	 * to perform such validation on the last 6 since the last 6 cannot
-	 * inherit any data from a descriptor after them.
+	 * of 6 fragments totals at least gso_size.
 	 */
 	nr_frags -= I40E_MAX_BUFFER_TXD - 2;
 	frag = &skb_shinfo(skb)->frags[0];
@@ -2654,8 +2652,7 @@ bool __i40e_chk_linearize(struct sk_buff *skb)
 		if (sum < 0)
 			return true;
 
-		/* use pre-decrement to avoid processing last fragment */
-		if (!--nr_frags)
+		if (!nr_frags--)
 			break;
 
 		sum -= skb_frag_size(stale++);

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

@@ -1832,9 +1832,7 @@ bool __i40evf_chk_linearize(struct sk_buff *skb)
 		return false;
 
 	/* We need to walk through the list and validate that each group
-	 * of 6 fragments totals at least gso_size.  However we don't need
-	 * to perform such validation on the last 6 since the last 6 cannot
-	 * inherit any data from a descriptor after them.
+	 * of 6 fragments totals at least gso_size.
 	 */
 	nr_frags -= I40E_MAX_BUFFER_TXD - 2;
 	frag = &skb_shinfo(skb)->frags[0];
@@ -1865,8 +1863,7 @@ bool __i40evf_chk_linearize(struct sk_buff *skb)
 		if (sum < 0)
 			return true;
 
-		/* use pre-decrement to avoid processing last fragment */
-		if (!--nr_frags)
+		if (!nr_frags--)
 			break;
 
 		sum -= skb_frag_size(stale++);