cgroup: Use local variable prop in restore_special_props()

For the purpose of separation this as a function, refactor
the code to use local variable prop, so the new function will have
(CgroupPropEntry *) type instead of (CgroupDirEntry *).

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>