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
* 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.