bpftool: replace return value PTR_ERR(NULL) with 0

There is no reasons to keep PTR_ERR() when kern_btf=NULL, let's just
return 0.
This also cleans this part of code from using libbpf_get_error().

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@industrialdiscipline.com>
Acked-by: Yonghong Song <yhs@fb.com>
Suggested-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20221120112515.38165-3-sahid.ferdjaoui@industrialdiscipline.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/struct_ops.c

@@ -63,10 +63,8 @@ static __s32 get_map_info_type_id(void)
 		return map_info_type_id;
 
 	kern_btf = get_btf_vmlinux();
-	if (libbpf_get_error(kern_btf)) {
-		map_info_type_id = PTR_ERR(kern_btf);
-		return map_info_type_id;
-	}
+	if (!kern_btf)
+		return 0;
 
 	map_info_type_id = btf__find_by_name_kind(kern_btf, "bpf_map_info",
 						  BTF_KIND_STRUCT);