cgroup: fix memory leak in cgroup_rm_cftypes()

The memory allocated in cgroup_add_cftypes() should be freed. The
effect of this bug is we leak a bit memory everytime we unload
cfq-iosched module if blkio cgroup is enabled.

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