perf bpf-loader: Add missing '*' for key_scan_pos

key_scan_pos is a pointer for getting scan position in
bpf__obj_config_map() for each BPF map configuration term,
but it's misused when error not happened.

Committer notes:

The point is that the only user of this is:

  tools/perf/util/parse-events.c
    err = bpf__config_obj(obj, term, parse_state->evlist, &error_pos);
      if (err) bpf__strerror_config_obj(obj, term, parse_state->evlist, &error_pos, err, errbuf, sizeof(errbuf));

And then:

int bpf__strerror_config_obj(struct bpf_object *obj __maybe_unused,
                             struct parse_events_term *term __maybe_unused,
                             struct evlist *evlist __maybe_unused,
                             int *error_pos __maybe_unused, int err,
                             char *buf, size_t size)
{
        bpf__strerror_head(err, buf, size);
        bpf__strerror_entry(BPF_LOADER_ERRNO__OBJCONF_MAP_TYPE,
                            "Can't use this config term with this map type");
        bpf__strerror_end(buf, size);
        return 0;
}

So this is infrastructure that Wang Nan put in place for providing
better error messages but that he ended up not using, so I'll apply the
fix, its correct even not fixing any real problem at this time.

Fixes: 066dacbf2a ("perf bpf: Add API to set values to map entries in a bpf object")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Cheng Jian <cj.chengjian@huawei.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Li Bin <huawei.libin@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Xie XiuQi <xiexiuqi@huawei.com>
Cc: bpf@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200520033216.48310-1-bobo.shaobowang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf-loader.c

@@ -1225,7 +1225,7 @@ bpf__obj_config_map(struct bpf_object *obj,
 out:
 	free(map_name);
 	if (!err)
-		key_scan_pos += strlen(map_opt);
+		*key_scan_pos += strlen(map_opt);
 	return err;
 }