e1000/e1000e: Fix leak in DMA error cleanup

If an error is encountered while mapping TX buffers, the driver should
unmap any buffers already mapped for that skb.

Because count is incremented after a successful mapping, it will always
match the correct number of unmappings needed when dma_error is reached.
Decrementing count before the while loop in dma_error causes an
off-by-one error. If any mapping was successful before an unsuccessful
mapping, exactly one DMA mapping would leak.

In these commits, a faulty while condition caused an infinite loop in
dma_error:
Commit 03b1320 ("e1000e: remove use of skb_dma_map from e1000e
driver")
Commit 602c055 ("e1000: remove use of skb_dma_map from e1000 driver")

Commit c1fa347 ("e1000/e1000e/igb/igbvf/ixgb/ixgbe: Fix tests of
unsigned in *_tx_map()") fixed the infinite loop, but introduced the
off-by-one error.

This issue may still exist in the igbvf driver, but I did not address it
in this patch.

Fixes: c1fa347 ("e1000/e1000e/igb/igbvf/ixgb/ixgbe: Fix tests of unsigned in *_tx_map()")
Assisted-by: Claude:claude-4.6-opus
Signed-off-by: Matt Vollrath <tactii@gmail.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: mvollrath

drivers/net/ethernet/intel/e1000/e1000_main.c

@@ -2952,8 +2952,6 @@ static int e1000_tx_map(struct e1000_adapter *adapter,
 dma_error:
 	dev_err(&pdev->dev, "TX DMA map failed\n");
 	buffer_info->dma = 0;
-	if (count)
-		count--;
 
 	while (count--) {
 		if (i == 0)

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -5652,8 +5652,6 @@ static int e1000_tx_map(struct e1000_ring *tx_ring, struct sk_buff *skb,
 dma_error:
 	dev_err(&pdev->dev, "Tx DMA map failed\n");
 	buffer_info->dma = 0;
-	if (count)
-		count--;
 
 	while (count--) {
 		if (i == 0)