IB/iser: Use separate buffers for the login request/response

The driver counted on the transactional nature of iSCSI login/text
flows and used the same buffer for both the request and the response.
We also went further and did DMA mapping only once, with
DMA_FROM_DEVICE, which violates the DMA mapping API.  Fix that by
using different buffers, one for requests and one for responses, and
use the correct DMA mapping direction for each.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/ulp/iser/iscsi_iser.h

@@ -257,7 +257,8 @@ struct iser_conn {
 	struct list_head	     conn_list;       /* entry in ig conn list */
 
 	char  			     *login_buf;
-	u64 			     login_dma;
+	char			     *login_req_buf, *login_resp_buf;
+	u64			     login_req_dma, login_resp_dma;
 	unsigned int 		     rx_desc_head;
 	struct iser_rx_desc	     *rx_descs;
 	struct ib_recv_wr	     rx_wr[ISER_MIN_POSTED_RX];