libbpf: Skip bpf_object__probe_loading for light skeleton

I'm getting the following error when running 'gen skeleton -L' as
regular user:

libbpf: Error in bpf_object__probe_loading():Operation not permitted(1).
Couldn't load trivial BPF program. Make sure your kernel supports BPF
(CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is set to big enough
value.

Fixes: 6723474373 ("libbpf: Generate loader program out of BPF ELF file.")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210521030653.2626513-1-sdf@google.com

tools/lib/bpf/libbpf.c

@@ -3971,6 +3971,9 @@ bpf_object__probe_loading(struct bpf_object *obj)
 	};
 	int ret;
 
+	if (obj->gen_loader)
+		return 0;
+
 	/* make sure basic loading works */
 
 	memset(&attr, 0, sizeof(attr));