use process thread group id for the process

in cgroupv2 we should use the cgroups.proc file when adding a process (https://www.man7.org/linux/man-pages/man7/cgroups.7.html).  The add_tasks function was writing to the cgroup.threads file which is only avaliable when in threaded mode.  In either case our intent is to add the process not the individual threads to we should use add_task_by_tgid. See kata-containers/cgroups-rs#104 for when this was added

Signed-off-by: James Sturtevant <jstur@microsoft.com>

Author: jsturtevant

crates/shim/src/cgroup.rs

@@ -64,7 +64,7 @@ pub fn add_task_to_cgroup(path: &str, pid: u32) -> Result<()> {
     let path = path.trim_start_matches('/');
 
     Cgroup::load(h, path)
-        .add_task(CgroupPid::from(pid as u64))
+        .add_task_by_tgid(CgroupPid::from(pid as u64))
         .map_err(other_error!(e, "add task to cgroup"))
 }
 
@@ -310,7 +310,7 @@ mod tests {
         assert!(cg.tasks().contains(&cg_id));
 
         // remove cgroup as possible
-        cg.remove_task(cg_id).unwrap();
+        cg.remove_task_by_tgid(cg_id).unwrap();
         cg.delete().unwrap()
     }