cgroup: nested cgroups aren't really supported

After some discussion with Serge Hallyn, it seems that the current
implementation of cgroup namespaces doesn't really support nesting. It's
not a quick fix, so let's disable this for now (not that it matters, since
probably nobody is nesting these anyways right now :)
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/namespaces.h

@@ -11,7 +11,7 @@
 #endif
 
 /* Nested namespaces are supported only for these types */
-#define CLONE_SUBNS	(CLONE_NEWNS | CLONE_NEWCGROUP)
+#define CLONE_SUBNS	(CLONE_NEWNS)
 
 struct ns_desc {
 	unsigned int	cflag;