update submodule to zfs-0.8.1

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/upstream b/upstream
index e34c3ee2fcbb320525a110c1811c557bc60479ff..63b88f7e223c1061c522762803b1431e7faba5b3 160000 (submodule)
--- a/upstream
+++ b/upstream
@@ -1 +1 @@
-Subproject commit e34c3ee2fcbb320525a110c1811c557bc60479ff
+Subproject commit 63b88f7e223c1061c522762803b1431e7faba5b3