Previously, the lua stats collection was building its own paths to the
cgroup files, which could be wrong depending on what --with-cgroup-pattern
was passed to configure. Fix it to use the get_cgroup_item api so it
always finds the files.

Remove cgroup_path_get since it is not used anymore.
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>