core: check fs type of BPFProgram= property path
Tests:
```
% stat --file-system --format="%T" /root/bpf/trivial/
bpf_fs
% systemd-nspawn -D/ --volatile=yes \
--property=BPFProgram=egress:/root/bpf/trivial/cgroup_skb_egress \
--quiet -- ping -c 5 -W 1 ::1
PING ::1(::1) 56 data bytes