blk-cgroup: don't update io stat for root cgroup
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2167245
Signed-off-by: Ming Lei ming.lei@redhat.com
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2167245
Signed-off-by: Ming Lei ming.lei@redhat.com