Fix cgroupv2 mem usage calculation to match docker cli

[sparrc]

"cache" memory stat no longer exists in cgroupv2.

docker cli subtracts "inactive_file" for the overall mem usage
calculation, so do the same for cgroupv2.

closes #3323

for reference also see moby/moby#40727

Summary

Testing

Agent code was instrumented to log the exact mem usage that it is sending to cloudwatch. This was then manually compared to what docker cli outputs before and after this change.

Note in the logging below that the mem usage "AFTER" this change more accurately matches the docker cli value of 51.85 MiB

src/github.com/aws/amazon-ecs-agent % docker stats f6bf24ec4486 --no-stream
CONTAINER ID   NAME                                             CPU %     MEM USAGE / LIMIT   MEM %     NET I/O       BLOCK I/O     PIDS
f6bf24ec4486   ecs-tomcat-sample-4-main1-f4e2c581ecd7d49c9201   0.05%     51.85MiB / 256MiB   20.25%    1.14kB / 0B   0B / 2.03MB   30

src/github.com/aws/amazon-ecs-agent % tail -f /var/log/ecs/ecs-agent.log   
level=info time=2022-08-29T23:00:50Z msg="BEFORE: 51.8906 mb    AFTER: 51.8516" module=utils_unix.go
level=info time=2022-08-29T23:00:51Z msg="BEFORE: 51.8906 mb    AFTER: 51.8516" module=utils_unix.go

also see #3323

New tests cover the changes: no

Description for the changelog

Bugfix: Fix cgroupv2 mem usage calculation to match docker cli

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.