scsi: iscsi: Fix inappropriate use of put_device()

kfree(conn) is called inside put_device(&conn->dev) which could lead to
use-after-free. In addition, device_unregister() should be used here rather
than put_deviceO().

Link: https://lore.kernel.org/r/20201120074852.31658-1-miaoqinglang@huawei.com
Fixes: f3c893e3 ("scsi: iscsi: Fail session and connection on transport registration failure")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_transport_iscsi.c

@@ -2313,7 +2313,9 @@ iscsi_create_conn(struct iscsi_cls_session *session, int dd_size, uint32_t cid)
 	return conn;
 
 release_conn_ref:
-	put_device(&conn->dev);
+	device_unregister(&conn->dev);
+	put_device(&session->dev);
+	return NULL;
 release_parent_ref:
 	put_device(&session->dev);
 free_conn: