perf bpf: Defer freeing string after possible strlen() on it

This was detected by the gcc in Fedora Rawhide's gcc:

  50    11.01 fedora:rawhide                : FAIL gcc version 12.0.1 20220205 (Red Hat 12.0.1-0) (GCC)
        inlined from 'bpf__config_obj' at util/bpf-loader.c:1242:9:
    util/bpf-loader.c:1225:34: error: pointer 'map_opt' may be used after 'free' [-Werror=use-after-free]
     1225 |                 *key_scan_pos += strlen(map_opt);
          |                                  ^~~~~~~~~~~~~~~
    util/bpf-loader.c:1223:9: note: call to 'free' here
     1223 |         free(map_name);
          |         ^~~~~~~~~~~~~~
    cc1: all warnings being treated as errors

So do the calculations on the pointer before freeing it.

Fixes: 04f9bf2bac ("perf bpf-loader: Add missing '*' for key_scan_pos")
Cc: Adrian Hunter <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang ShaoBo <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/bpf-loader.c

@@ -1220,9 +1220,10 @@ bpf__obj_config_map(struct bpf_object *obj,
 	pr_debug("ERROR: Invalid map config option '%s'\n", map_opt);
 	err = -BPF_LOADER_ERRNO__OBJCONF_MAP_OPT;
 out:
-	free(map_name);
 	if (!err)
 		*key_scan_pos += strlen(map_opt);
+
+	free(map_name);
 	return err;
 }