core: check fs type of BPFProgram= property path

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

--- ::1 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4110ms
```

```
% stat --file-system --format='%T' /root/meh
btrfs

% systemd-nspawn -D/ --volatile=yes --property=BPFProgram=egress:/root/meh
--quiet -- ping -c 5 -W 1 ::1
```
sudo ./build/systemd-nspawn \
-D/ --volatile=yes --property=BPFProgram=egress:/home/hex --quiet -- \
ping -c 1 -W 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.017 ms

--- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms