cgroup: freezer action must be NOP when cgroup v2 freezer is not available
    
    Low-level cgroup freezer state manipulation is invoked directly from the
    job engine when we are about to execute the job in order to make sure
    the unit is not frozen and job execution is not blocked because of
    that.
    
    Currently with cgroup v1 we would needlessly do a bunch of work in the
    function and even falsely update the freezer state. Don't do any of this
    and skip the function silently when v2 freezer is not available.
    
    Following bug is fixed by this commit,
    
    $ systemd-run --unit foo.service /bin/sleep infinity
    $ systemctl restart foo.service
    $ systemctl show -p FreezerState foo.service
    
    Before (cgroup v1, i.e. full "legacy" mode):
    FreezerState=thawing
    
    After:
    FreezerState=running
    
    (cherry picked from commit 9a1e90aee556b7a30d87553a891a4175ae77ed68)
    
    Resolves: #1868831
    
    patch_name: 0435-cgroup-freezer-action-must-be-NOP-when-cgroup-v2-fre.patch
    present_in_specfile: true
    location_in_specfile: 435
    squash_commits: true