Always use cgroup root of current process

Because for host PID namespace /proc/1/cgroup can point to whole other
world of cgroups.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

libcontainer/cgroups/fs/apply_raw.go

@@ -236,7 +236,7 @@ func getCgroupData(c *configs.Cgroup, pid int) (*data, error) {
 }
 
 func (raw *data) parent(subsystem, mountpoint, src string) (string, error) {
-	initPath, err := cgroups.GetInitCgroupDir(subsystem)
+	initPath, err := cgroups.GetThisCgroupDir(subsystem)
 	if err != nil {
 		return "", err
 	}