cgroup: collect path of cgroup when running a container in a container

Path of cgroup is wrong when running a container in a container. Use
the root path of mountinfo fetched from /proc/$(shim_pid)/mountinfo
to trim the path obtained from /proc/self/mountinfo.

Fixes: #131

Signed-off-by: 乔琛 10307740 <qiao.chen@zte.com.cn>

src/memory.rs

@@ -540,10 +540,10 @@ impl ControllerInternal for MemController {
 
 impl MemController {
     /// Contructs a new `MemController` with `root` serving as the root of the control group.
-    pub fn new(root: PathBuf, v2: bool) -> Self {
+    pub fn new(point: PathBuf, root: PathBuf, v2: bool) -> Self {
         Self {
-            base: root.clone(),
-            path: root,
+            base: root,
+            path: point,
             v2,
         }
     }