bpf: fix compiler warning with CONFIG_MODULES=n

With CONFIG_MODULES=n, the following compiler warning occurs:
  /data/users/yhs/work/net-next/kernel/trace/bpf_trace.c:605:13: warning:
      ‘do_bpf_send_signal’ defined but not used [-Wunused-function]
  static void do_bpf_send_signal(struct irq_work *entry)

The __init function send_signal_irq_work_init(), which calls
do_bpf_send_signal(), is defined under CONFIG_MODULES. Hence,
when CONFIG_MODULES=n, nobody calls static function do_bpf_send_signal(),
hence the warning.

The init function send_signal_irq_work_init() should work without
CONFIG_MODULES. Moving it out of CONFIG_MODULES
code section fixed the compiler warning, and also make bpf_send_signal()
helper work without CONFIG_MODULES.

Fixes: 8b401f9e ("bpf: implement bpf_send_signal() helper")
Reported-By: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/trace/bpf_trace.c

@@ -1431,6 +1431,20 @@ int bpf_get_perf_event_info(const struct perf_event *event, u32 *prog_id,
 	return err;
 }
 
+static int __init send_signal_irq_work_init(void)
+{
+	int cpu;
+	struct send_signal_irq_work *work;
+
+	for_each_possible_cpu(cpu) {
+		work = per_cpu_ptr(&send_signal_work, cpu);
+		init_irq_work(&work->irq_work, do_bpf_send_signal);
+	}
+	return 0;
+}
+
+subsys_initcall(send_signal_irq_work_init);
+
 #ifdef CONFIG_MODULES
 static int bpf_event_notify(struct notifier_block *nb, unsigned long op,
 			    void *module)
@@ -1478,18 +1492,5 @@ static int __init bpf_event_init(void)
 	return 0;
 }
 
-static int __init send_signal_irq_work_init(void)
-{
-	int cpu;
-	struct send_signal_irq_work *work;
-
-	for_each_possible_cpu(cpu) {
-		work = per_cpu_ptr(&send_signal_work, cpu);
-		init_irq_work(&work->irq_work, do_bpf_send_signal);
-	}
-	return 0;
-}
-
 fs_initcall(bpf_event_init);
-subsys_initcall(send_signal_irq_work_init);
 #endif /* CONFIG_MODULES */