sbin/init.wrapper: remove processes from subcgroups before deleting

[jepio]

sbin/init.wrapper: remove processes from subcgroups before deleting

In an attempt to prevent the cgroup from being busy when we unbind controllers
from it.

Will be cherry-picked to flatcar-3139.

How to use

[ describe what reviewers need to do in order to validate this PR ]

Testing done

http://jenkins.infra.kinvolk.io:8080/job/os/job/manifest/5007/cldsv/

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)

[krnowak]

Hopefully the pids of the processes spawned during this cleanup won't end up in any of the cgroup.procs that we are trying to clean up.

[jepio]

This loop:

  find /sys/fs/cgroup/*/ -name cgroup.procs -exec grep . {} \; | \
  while read pid; do
    echo "$pid" >/sys/fs/cgroup/cgroup.procs 2>/dev/null
  done

also covers "self", thats accounted for :)