cg: don't save cgroup directories twice

The path in cc->path here always has a "/" prefix since it comes from
/proc/$pid/cgroup. The additional / confuses the string slinging because ftw()
normalizes paths to have one "/" when we start traversing a subdirectory.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>