ioat: implement a private tx_list

Drop ioatdma's use of tx_list from struct dma_async_tx_descriptor in
preparation for removal of this field.

Cc: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/dma/ioat/dma.h b/drivers/dma/ioat/dma.h
index d9d6a7e..8966fa5 100644
--- a/drivers/dma/ioat/dma.h
+++ b/drivers/dma/ioat/dma.h
@@ -157,7 +157,7 @@
  * struct ioat_desc_sw - wrapper around hardware descriptor
  * @hw: hardware DMA descriptor
  * @node: this descriptor will either be on the free list,
- *     or attached to a transaction list (async_tx.tx_list)
+ *     or attached to a transaction list (tx_list)
  * @txd: the generic software descriptor for all engines
  * @id: identifier for debug
  */
@@ -165,6 +165,7 @@
 	struct ioat_dma_descriptor *hw;
 	struct list_head node;
 	size_t len;
+	struct list_head tx_list;
 	struct dma_async_tx_descriptor txd;
 	#ifdef DEBUG
 	int id;