samples/kretprobes: Fix return value if register_kretprobe() failed

Use the actual return value instead of always -1 if register_kretprobe()
failed.

E.g. without this patch:

 # insmod samples/kprobes/kretprobe_example.ko func=no_such_func
 insmod: ERROR: could not insert module samples/kprobes/kretprobe_example.ko: Operation not permitted

With this patch:

 # insmod samples/kprobes/kretprobe_example.ko func=no_such_func
 insmod: ERROR: could not insert module samples/kprobes/kretprobe_example.ko: Unknown symbol in module

Link: https://lkml.kernel.org/r/1635213091-24387-2-git-send-email-yangtiezhu@loongson.cn

Fixes: 804defea ("Kprobes: move kprobe examples to samples/")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/kprobes/kretprobe_example.c

@@ -86,7 +86,7 @@ static int __init kretprobe_init(void)
 	ret = register_kretprobe(&my_kretprobe);
 	if (ret < 0) {
 		pr_err("register_kretprobe failed, returned %d\n", ret);
-		return -1;
+		return ret;
 	}
 	pr_info("Planted return probe at %s: %p\n",
 			my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr);