core/cgroup: make sure deserialized accounting data is not voided

core/cgroup: make sure deserialized accounting data is not voided

Currently, cgroup_path is (de-)serialized after all the cached
accounting data. This is bogus though, since unit_set_cgroup_path()
destroys the CGroupRuntime object and starts afresh, discarding
all deserialized values. This matters especially for IP accounting,
whose BPF maps get recreated on reload/reexec and the previous values
are exclusively retrievable from deserialization. Let's hence swap things
around and serialize cgroup_path first, accounting data only afterwards.