oci_linux: fix working set calculation for cgroupv2
oci_linux: fix working set calculation for cgroupv2

It's done in a same manner as for v1, except for cgroupv2 there's no
total_* counters in memory.stat. My assumption is the counters now
include subcgroup counters, although I am only 95% sure about that
even after digging into the cgroupv2 docs.

Here's an emprirical way I used to check the above assumption:

	find /sys/fs/cgroup/system.slice/ -name memory.stat -type f \
		| xargs grep ^inactive_file 2>/dev/null \
		| awk '$2 > 0 {printf "%20d %s\n",$2,$1}' \
		| sort -nr

It shows that the top-level's cgroup value is higher than that of
sub-cgroups for both system.slice and user.slice.

PS I have also checked it is done the same way in containerd/cri.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

kubernetes-incubator/ocid,nalind/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/ocid