ib_isert: Add max_send_sge=2 minimum for control PDU responses

This patch adds a max_send_sge=2 minimum in isert_conn_setup_qp()
to ensure outgoing control PDU responses with tx_desc->num_sge=2
are able to function correctly.

This addresses a bug with RDMA hardware using dev_attr.max_sge=3,
that in the original code with the ConnectX-2 work-around would
result in isert_conn->max_sge=1 being negotiated.

Originally reported by Chris with ocrdma driver.

Reported-by: Chris Moore <Chris.Moore@emulex.com>
Tested-by: Chris Moore <Chris.Moore@emulex.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Cc: <stable@vger.kernel.org> # 3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/infiniband/ulp/isert/ib_isert.c

@@ -115,9 +115,12 @@ isert_conn_setup_qp(struct isert_conn *isert_conn, struct rdma_cm_id *cma_id,
 	attr.cap.max_recv_wr = ISERT_QP_MAX_RECV_DTOS;
 	/*
 	 * FIXME: Use devattr.max_sge - 2 for max_send_sge as
-	 * work-around for RDMA_READ..
+	 * work-around for RDMA_READs with ConnectX-2.
+	 *
+	 * Also, still make sure to have at least two SGEs for
+	 * outgoing control PDU responses.
 	 */
-	attr.cap.max_send_sge = device->dev_attr.max_sge - 2;
+	attr.cap.max_send_sge = max(2, device->dev_attr.max_sge - 2);
 	isert_conn->max_sge = attr.cap.max_send_sge;
 
 	attr.cap.max_recv_sge = 1;