perf stat: Support old kernels for bperf cgroup counting

The recent change in the cgroup will break the backward compatiblity in
the BPF program.  It should support both old and new kernels using BPF
CO-RE technique.

Like the task_struct->__state handling in the offcpu analysis, we can
check the field name in the cgroup struct.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>