cgroup: update error handling in cgroup_populate_dir()

cgroup_populate_dir() didn't use to check whether the actual file
creations were successful and could return success with only subset of
the requested files created, which is nasty.

This patch udpates cgroup_populate_dir() so that it either succeeds
with all files or fails with no file.

v2: The original patch also converted for_each_root_subsys() usages to
    for_each_subsys() without explaining why.  That part has been
    moved to a separate patch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>