cgroup: make cgroup_path() not print double slashes

While reimplementing cgroup_path(), 65dff759 ("cgroup: fix
cgroup_path() vs rename() race") introduced a bug where the path of a
non-root cgroup would have two slahses at the beginning, which is
caused by treating the root cgroup which has the name '/' like
non-root cgroups.

 $ grep systemd /proc/self/cgroup
 1:name=systemd://user/root/1



Fix it by special casing root cgroup case and not looping over it in
the normal path.

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