libbpf: Don't enforce unnecessary verifier log restrictions on libbpf side

This basically prevents any forward compatibility. And we either way
just return -EINVAL, which would otherwise be returned from bpf()
syscall anyways.

Similarly, drop enforcement of non-NULL log_buf when log_level > 0. This
won't be true anymore soon.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-5-andrii@kernel.org