rustjail: fix the issue of invalid cgroup_parent path

The cgroup_parent path is expected to be abolute path,
if it was passed a relative path, change it to be an
absolute path.

Fixes: kata-containers#336

Signed-off-by: fupan.lfp <fupan.lfp@antfin.com>

src/agent/rustjail/src/cgroups/fs/mod.rs

@@ -1388,9 +1388,11 @@ impl CgroupManager for Manager {
 impl Manager {
     pub fn new(cpath: &str) -> Result<Self> {
         let mut m = HashMap::new();
+        let mut cgroup_path = String::from("");
 
         if !cpath.starts_with('/') {
-            return Err(nix::Error::Sys(Errno::EINVAL).into());
+            cgroup_path.push('/');
+            cgroup_path.push_str(cpath);
         }
 
         let paths = get_paths()?;
@@ -1404,9 +1406,9 @@ impl Manager {
             }
 
             let p = if value == "/" {
-                format!("{}{}", mnt.unwrap(), cpath)
+                format!("{}{}", mnt.unwrap(), cgroup_path)
             } else {
-                format!("{}{}{}", mnt.unwrap(), value, cpath)
+                format!("{}{}{}", mnt.unwrap(), value, cgroup_path)
             };
 
             m.insert(key.to_string(), p);