cgroupfs: need the full path to setup cpuset

Function file_exists() needs a absolute full path, but we are using
current_entire_path which is not. It will get the wrong result from
file_exists() and case Segmentation fault when we fopen a non-exist
file and try to fscanf from it.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>