cgroup.c: clean up for handle_cgroup_settings

Clean up the nesting if, make the logic similar for memory
and cpuset, and the error message should sent from inside,
for better extendibility.

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