bpftool: fix error message when function can't register struct_ops

It is expected that errno be passed to strerror(). 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>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20221120112515.38165-4-sahid.ferdjaoui@industrialdiscipline.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/struct_ops.c

@@ -511,10 +511,9 @@ static int do_register(int argc, char **argv)
 			continue;
 
 		link = bpf_map__attach_struct_ops(map);
-		if (libbpf_get_error(link)) {
+		if (!link) {
 			p_err("can't register struct_ops %s: %s",
-			      bpf_map__name(map),
-			      strerror(-PTR_ERR(link)));
+			      bpf_map__name(map), strerror(errno));
 			nr_errs++;
 			continue;
 		}