cgroup: unify cgroup_write_X64() and cgroup_write_string()
cgroup_write_X64() and cgroup_write_string() both implement about the
same buffering logic. Unify the two into cgroup_file_write() which
always allocates dynamic buffer for simplicity and uses kstrto*()
instead of simple_strto*().
This patch doesn't make any visible behavior changes except for
possibly different error value from kstrsto*().
Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Li Zefan <lizefan@huawei.com>