scsi: iscsi_tcp: Check that sock is valid before iscsi_set_param()

The validity of sock should be checked before assignment to avoid incorrect
values. Commit 57569c3 ("scsi: iscsi: iscsi_tcp: Fix null-ptr-deref
while calling getpeername()") introduced this change which may lead to
inconsistent values of tcp_sw_conn->sendpage and conn->datadgst_en.

Fix the issue by moving the position of the assignment.

Fixes: 57569c3 ("scsi: iscsi: iscsi_tcp: Fix null-ptr-deref while calling getpeername()")
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Link: https://lore.kernel.org/r/20230329071739.2175268-1-zhongjinghua@huaweicloud.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/iscsi_tcp.c

@@ -771,13 +771,12 @@ static int iscsi_sw_tcp_conn_set_param(struct iscsi_cls_conn *cls_conn,
 		iscsi_set_param(cls_conn, param, buf, buflen);
 		break;
 	case ISCSI_PARAM_DATADGST_EN:
-		iscsi_set_param(cls_conn, param, buf, buflen);
-
 		mutex_lock(&tcp_sw_conn->sock_lock);
 		if (!tcp_sw_conn->sock) {
 			mutex_unlock(&tcp_sw_conn->sock_lock);
 			return -ENOTCONN;
 		}
+		iscsi_set_param(cls_conn, param, buf, buflen);
 		tcp_sw_conn->sendpage = conn->datadgst_en ?
 			sock_no_sendpage : tcp_sw_conn->sock->ops->sendpage;
 		mutex_unlock(&tcp_sw_conn->sock_lock);