ionic: linearize tso skb with too many frags

We were linearizing non-TSO skbs that had too many frags, but
we weren't checking number of frags on TSO skbs.  This could
lead to a bad page reference when we received a TSO skb with
more frags than the Tx descriptor could support.

v2: use gso_segs rather than yet another division
    don't rework the check on the nr_frags

Fixes: 0f3154e6 ("ionic: Add Tx and Rx handling")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/pensando/ionic/ionic_txrx.c

@@ -1079,15 +1079,17 @@ static int ionic_tx_descs_needed(struct ionic_queue *q, struct sk_buff *skb)
 {
 	int sg_elems = q->lif->qtype_info[IONIC_QTYPE_TXQ].max_sg_elems;
 	struct ionic_tx_stats *stats = q_to_tx_stats(q);
+	int ndescs;
 	int err;
 
-	/* If TSO, need roundup(skb->len/mss) descs */
+	/* Each desc is mss long max, so a descriptor for each gso_seg */
 	if (skb_is_gso(skb))
-		return (skb->len / skb_shinfo(skb)->gso_size) + 1;
+		ndescs = skb_shinfo(skb)->gso_segs;
+	else
+		ndescs = 1;
 
-	/* If non-TSO, just need 1 desc and nr_frags sg elems */
 	if (skb_shinfo(skb)->nr_frags <= sg_elems)
-		return 1;
+		return ndescs;
 
 	/* Too many frags, so linearize */
 	err = skb_linearize(skb);
@@ -1096,8 +1098,7 @@ static int ionic_tx_descs_needed(struct ionic_queue *q, struct sk_buff *skb)
 
 	stats->linearize++;
 
-	/* Need 1 desc and zero sg elems */
-	return 1;
+	return ndescs;
 }
 
 static int ionic_maybe_stop_tx(struct ionic_queue *q, int ndescs)