ceph-object-corpus: update submodule

Update the ceph-object-corpus submodule to pick up the change to
mark cls_rbd_snap as forward incompatible since nautilus. This change
allows us to update ceph-dencoder to allocate fresh instances for each
decode operation instead of reusing existing ones. The uncoming
change in ceph-dencoder will allow us to identify the potential
compatibility break early.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

diff --git a/ceph-object-corpus b/ceph-object-corpus
index 6b15dbab5629b79bc9364054cf1d3ca6c01d9af2..9670a0ef3c10bb2afa1ed4f75cbdfdc954a170cb 160000 (submodule)
--- a/ceph-object-corpus
+++ b/ceph-object-corpus
@@ -1 +1 @@
-Subproject commit 6b15dbab5629b79bc9364054cf1d3ca6c01d9af2
+Subproject commit 9670a0ef3c10bb2afa1ed4f75cbdfdc954a170cb