bpf: do not freeze if bpf lsm fails to set up

BPF LSM is cgroup unaware and it's set up is happening in core manager.
It occures that the current implementation is too restrictive and causes
pid 1 to freeze.
Instead:
* in bpf_lsm_setup set manager->restrict_fs pointer last,
so it is an indicator that the set up was successful
* check for manager->restrict_fs before applying unit options