update upstream to 1.28

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

diff --git a/upstream b/upstream
index a40c3549012f3e418626dec02c8999ac0aa83d04..6b8ad835cda1e05741cfb7f664ae630dee74ee2f 160000 (submodule)
--- a/upstream
+++ b/upstream
@@ -1 +1 @@
-Subproject commit a40c3549012f3e418626dec02c8999ac0aa83d04
+Subproject commit 6b8ad835cda1e05741cfb7f664ae630dee74ee2f