core: fix bpf-foreign cg controller realization
    
    Requiring /sys/fs/bpf path to be a mount point at the moment of cgroup
    controllers realization does more harm than good, because:
    * Realization happens early on boot, the mount point may not be ready at
    the time. That happens if mounts are made by a .mount unit (the issue we
    encountered).
    * BPF filesystem may be mounted on another point.
    
    Remove the check. Instead verify that path provided by BPFProgram= is
    within BPF fs when unit properties are parsed.
    
    Split in two commits for simple backport.