bpf: check if lsm link ptr is libbpf error

bpf: check if lsm link ptr is libbpf error

BPF_RAW_TRACEPOINT_OPEN is expected to work only on x86 and x86_64,
since BPF trampoline is implemented only on these architectures.

Attach probing by bpf_program__attach_lsm already happens in
`bpf_lsm_supported`. The resulting pointer can store libbpf error and
that is the case for unsupported architectures.
Add libbpf error check to `bpf_lsm_supported` so execution does not
reach the point where unit startup fails.