cgroup: make memory.{swappiness, use_hierarchy} special properties

In all kernels, memory.use_hierarchy must be written before there are child
cgroups. In 3.11, memory.swappiness must be written before there are child
cgroups, so let's consider it a "special" property for all kernels.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cgroup.c

@@ -934,6 +934,8 @@ static const char *special_props[] = {
 	"cpuset.cpus",
 	"cpuset.mems",
 	"memory.kmem.limit_in_bytes",
+	"memory.swappiness",
+	"memory.use_hierarchy",
 	NULL,
 };