Before this patch check is happened only in case when we try to find
cgroup at cgroup2 mount point.

v2:
  - add Fixes line before Signed-off-by (David Ahern)

Fixes: d5e6ee0d ("ss: introduce cgroup2 cache and helper functions")
Signed-off-by: Dmitry Yakunin <zeil@yandex-team.ru>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>