cgroup: don't overwrite the offset value in a loop (v2)

prepare_cgroup_dirs() gets a path and an offset.
Then we add substrings to the source string and handle them.

v2: fix one more place in prepare_cgroup_dir_properties()

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>