perf symbols: Stop using vmlinux files with no symbols

Fail if the kernel image contains no symbol, allowing using other images
in the vmlinux search path that may have a usable symtab.

Acked-by: Masami Hiramatsu <[e-mail]>
Cc: [e-mail]
Cc: Francis Moreau <[e-mail]>
Cc: Franck Bui-Huu <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
LPU-Reference: <[e-mail]>
Signed-off-by: Franck Bui-Huu <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/probe-event.c

···
 		if (dso__load_vmlinux(dso, map, vmlinux_name, NULL) <= 0)
 			return NULL;
 	} else {
-		if (dso__load_vmlinux_path(dso, map, NULL) < 0) {
+		if (dso__load_vmlinux_path(dso, map, NULL) <= 0) {
 			pr_debug("Failed to load kernel map.\n");
 			return NULL;
 		}