cgroup: reimplement reading "cgroup.procs" on cgroup v2

On v1, "tasks" and "cgroup.procs" are expected to be sorted which
makes the implementation expensive and unnecessarily complicated
involving result cache management.

v2 doesn't have the sorting requirement, so it can just iterate and
print processes one by one.  seq_files are either read sequentially or
reset to position zero, so the implementation doesn't even need to
worry about seeking.

This keeps the css_task_iter across multiple read(2) calls and
migrations of new processes always append won't miss processes which
are newly migrated in before each read(2).

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