libct/cg/sd/v2: call initPath from Path

Sometimes Path() is called before m.path is initialized (in particular,
this happens from (*linuxContainer).newInitConfig), so we do need to
make sure to call initPath.

This fixes the following integration tests (for cgroup v2 + systemd case,
currently not enabled -- to be enabled by further commits):

 *  runc run (blkio weight)
 * runc run (cgroupv2 mount inside container)

Fixes: ff692f2 ("Fix cgroup2 mount for rootless case")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/v2.go

@@ -317,6 +317,7 @@ func (m *unifiedManager) Destroy() error {
 }
 
 func (m *unifiedManager) Path(_ string) string {
+	_ = m.initPath()
 	return m.path
 }