cgroup: fix cgroup_add_cftypes() error handling

cgroup_add_cftypes() uses cgroup_cfts_commit() to actually create the
files; however, both functions ignore actual file creation errors and
just assume success.  This can lead to, for example, blkio hierarchy
with some of the cgroups with only subset of interface files populated
after cfq-iosched is loaded under heavy memory pressure, which is
nasty.

This patch updates cgroup_cfts_commit() and cgroup_add_cftypes() to
guarantee that all files are created on success and no file is created
on failure.

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