cgroups/fs/path: optimize

The result of cgroupv1.FindCgroupMountpoint() call (which is relatively
expensive) is only used in case raw.innerPath is absolute, so it only
makes sense to call it in that case.

This drastically reduces the number of calls to FindCgroupMountpoint
during container start (from 116 to 62 in my setup).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/fs/fs.go

@@ -359,14 +359,14 @@ func getCgroupData(c *configs.Cgroup, pid int) (*cgroupData, error) {
 }
 
 func (raw *cgroupData) path(subsystem string) (string, error) {
-	mnt, err := cgroups.FindCgroupMountpoint(raw.root, subsystem)
-	// If we didn't mount the subsystem, there is no point we make the path.
-	if err != nil {
-		return "", err
-	}
-
 	// If the cgroup name/path is absolute do not look relative to the cgroup of the init process.
 	if filepath.IsAbs(raw.innerPath) {
+		mnt, err := cgroups.FindCgroupMountpoint(raw.root, subsystem)
+		// If we didn't mount the subsystem, there is no point we make the path.
+		if err != nil {
+			return "", err
+		}
+
 		// Sometimes subsystems can be mounted together as 'cpu,cpuacct'.
 		return filepath.Join(raw.root, filepath.Base(mnt), raw.innerPath), nil
 	}