net: enetc: fix the off-by-one issue in enetc_map_tx_buffs()

When a DMA mapping error occurs while processing skb frags, it will free
one more tx_swbd than expected, so fix this off-by-one issue.

Fixes: d4fd040 ("enetc: Introduce basic PF and VF ENETC ethernet drivers")
Cc: stable@vger.kernel.org
Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Suggested-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Link: https://patch.msgid.link/20250224111251.1061098-2-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: WeiFang-NXP

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -167,6 +167,24 @@ static bool enetc_skb_is_tcp(struct sk_buff *skb)
 	return skb->csum_offset == offsetof(struct tcphdr, check);
 }
 
+/**
+ * enetc_unwind_tx_frame() - Unwind the DMA mappings of a multi-buffer Tx frame
+ * @tx_ring: Pointer to the Tx ring on which the buffer descriptors are located
+ * @count: Number of Tx buffer descriptors which need to be unmapped
+ * @i: Index of the last successfully mapped Tx buffer descriptor
+ */
+static void enetc_unwind_tx_frame(struct enetc_bdr *tx_ring, int count, int i)
+{
+	while (count--) {
+		struct enetc_tx_swbd *tx_swbd = &tx_ring->tx_swbd[i];
+
+		enetc_free_tx_frame(tx_ring, tx_swbd);
+		if (i == 0)
+			i = tx_ring->bd_count;
+		i--;
+	}
+}
+
 static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 {
 	bool do_vlan, do_onestep_tstamp = false, do_twostep_tstamp = false;
@@ -372,13 +390,7 @@ static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 dma_err:
 	dev_err(tx_ring->dev, "DMA map error");
 
-	do {
-		tx_swbd = &tx_ring->tx_swbd[i];
-		enetc_free_tx_frame(tx_ring, tx_swbd);
-		if (i == 0)
-			i = tx_ring->bd_count;
-		i--;
-	} while (count--);
+	enetc_unwind_tx_frame(tx_ring, count, i);
 
 	return 0;
 }