ioat: fix 'ack' handling, driver must ensure that 'ack' is zero

Initialize 'ack' to zero in case the descriptor has been recycled.

Prevents "kernel BUG at crypto/async_tx/async_xor.c:185!"
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Acked-by: NShannon Nelson <shannon.nelson@intel.com>
Cc: stable@kernel.org

drivers/dma/ioat_dma.c

@@ -714,6 +714,7 @@ static struct dma_async_tx_descriptor *ioat1_dma_prep_memcpy(
 		new->len = len;
 		new->dst = dma_dest;
 		new->src = dma_src;
+		new->async_tx.ack = 0;
 		return &new->async_tx;
 	} else
 		return NULL;
@@ -741,6 +742,7 @@ static struct dma_async_tx_descriptor *ioat2_dma_prep_memcpy(
 		new->len = len;
 		new->dst = dma_dest;
 		new->src = dma_src;
+		new->async_tx.ack = 0;
 		return &new->async_tx;
 	} else
 		return NULL;