cgroup: replace task_cgroup_path_from_hierarchy() with task_cgroup_path()

cgroup: replace task_cgroup_path_from_hierarchy() with task_cgroup_path()

task_cgroup_path_from_hierarchy() was added for the planned new users
and none of the currently planned users wants to know about multiple
hierarchies.  This patch drops the multiple hierarchy part and makes
it always return the path in the first non-dummy hierarchy.

As unified hierarchy will always have id 1, this is guaranteed to
return the path for the unified hierarchy if mounted; otherwise, it
will return the path from the hierarchy which happens to occupy the
lowest hierarchy id, which will usually be the first hierarchy mounted
after boot.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Jan Kaluža <jkaluza@redhat.com>
Change-Id: Iaa199f7332f01a03f791def776b5403f6fa459b3
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=913ffdb54366f94eec65c656cae8c6e00e1ab1b0
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>