libbpf: don't fail when feature probing fails

Otherwise libbpf is unusable from unprivileged process with
kernel.kernel.unprivileged_bpf_disabled=1.
All I get is EPERM from the probes, even if I just want to
open an ELF object and look at what progs/maps it has.

Instead of dying on probes, let's just pr_debug the error and
try to continue.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1696,7 +1696,7 @@ bpf_object__probe_caps(struct bpf_object *obj)
 	for (i = 0; i < ARRAY_SIZE(probe_fn); i++) {
 		ret = probe_fn[i](obj);
 		if (ret < 0)
-			return ret;
+			pr_debug("Probe #%d failed with %d.\n", i, ret);
 	}
 
 	return 0;