perf probe: Warn unmatched function filter correctly

Warn unmatched function filter correctly instead of warning
"symbol-loading error", since that can be a filter issue.

From the technical point of view, this adds a filter chech in map__load
and if there is a filter, it returns -2 (filter-out), instead of -1
(error), and perf-probe checks it and change message.

E.g. without this fix:

# perf probe -F rt_sp*
no symbols found in [kernel.kallsyms], maybe install a debug package?
Failed to load symbols in kernel

With this fix:

# perf probe -F rt_sp*
no symbols passed the given filter.
Failed to find symbols matched to "rt_sp*"
Error: Failed to show functions.

Reported-and-Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Signed-off-by: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/146885835596.16106.2293540792775552481.stgit@devbox
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/map.c

···
 			pr_warning("%.*s was updated (is prelink enabled?). "
 				"Restart the long running apps that use it!\n",
 				   (int)real_len, name);
+		} else if (filter) {
+			pr_warning("no symbols passed the given filter.\n");
+			return -2;	/* Empty but maybe by the filter */
 		} else {
 			pr_warning("no symbols found in %s, maybe install "
 				   "a debug package?\n", name);

tools/perf/util/probe-event.c

···
 
 	/* Load symbols with given filter */
 	available_func_filter = _filter;
-	if (map__load(map, filter_available_functions)) {
-		pr_err("Failed to load symbols in %s\n", (target) ? : "kernel");
+	ret = map__load(map, filter_available_functions);
+	if (ret) {
+		if (ret == -2) {
+			char *str = strfilter__string(_filter);
+			pr_err("Failed to find symbols matched to \"%s\"\n",
+			       str);
+			free(str);
+		} else
+			pr_err("Failed to load symbols in %s\n",
+			       (target) ? : "kernel");
 		goto end;
 	}
 	if (!dso__sorted_by_name(map->dso, map->type))