sfc: Fix DMA mapping cleanup in case of an error in TSO

We need buffer->len to remain valid to work out the correct address to
be unmapped.  We therefore need to clear buffer->len after the unmap
operation.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sfc/tx.c

@@ -821,8 +821,6 @@ static void efx_enqueue_unwind(struct efx_tx_queue *tx_queue)
 					   EFX_TXQ_MASK];
 		efx_tsoh_free(tx_queue, buffer);
 		EFX_BUG_ON_PARANOID(buffer->skb);
-		buffer->len = 0;
-		buffer->continuation = true;
 		if (buffer->unmap_len) {
 			unmap_addr = (buffer->dma_addr + buffer->len -
 				      buffer->unmap_len);
@@ -836,6 +834,8 @@ static void efx_enqueue_unwind(struct efx_tx_queue *tx_queue)
 					       PCI_DMA_TODEVICE);
 			buffer->unmap_len = 0;
 		}
+		buffer->len = 0;
+		buffer->continuation = true;
 	}
 }