zsmalloc: fix zs_init cpu notifier error handling

Mahendran Ganesh reported that zpool-enabled zsmalloc should not call
zpool_unregister_driver() from zs_init() if cpu notifier registration has
failed, because error handling is performed before we register the driver
via zpool_register_driver() call.

Factor out cpu notifier registration and unregistration code and fix
zs_init() error handling.

link: http://lkml.iu.edu//hypermail/linux/kernel/1411.1/04156.html
[akpm@linux-foundation.org: squash bogus gcc warning]
[akpm@linux-foundation.org: use __init and __exit]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Mahendran Ganesh <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -881,14 +881,10 @@ static struct notifier_block zs_cpu_nb = {
 	.notifier_call = zs_cpu_notifier
 };
 
-static void zs_exit(void)
+static void zs_unregister_cpu_notifier(void)
 {
 	int cpu;
 
-#ifdef CONFIG_ZPOOL
-	zpool_unregister_driver(&zs_zpool_driver);
-#endif
-
 	cpu_notifier_register_begin();
 
 	for_each_online_cpu(cpu)
@@ -898,31 +894,44 @@ static void zs_exit(void)
 	cpu_notifier_register_done();
 }
 
-static int zs_init(void)
+static int zs_register_cpu_notifier(void)
 {
-	int cpu, ret;
+	int cpu, uninitialized_var(ret);
 
 	cpu_notifier_register_begin();
 
 	__register_cpu_notifier(&zs_cpu_nb);
 	for_each_online_cpu(cpu) {
 		ret = zs_cpu_notifier(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
-		if (notifier_to_errno(ret)) {
-			cpu_notifier_register_done();
-			goto fail;
-		}
+		if (notifier_to_errno(ret))
+			break;
 	}
 
 	cpu_notifier_register_done();
+	return notifier_to_errno(ret);
+}
 
+static void __exit zs_exit(void)
+{
 #ifdef CONFIG_ZPOOL
-	zpool_register_driver(&zs_zpool_driver);
+	zpool_unregister_driver(&zs_zpool_driver);
 #endif
+	zs_unregister_cpu_notifier();
+}
+
+static int __init zs_init(void)
+{
+	int ret = zs_register_cpu_notifier();
+
+	if (ret) {
+		zs_unregister_cpu_notifier();
+		return ret;
+	}
 
+#ifdef CONFIG_ZPOOL
+	zpool_register_driver(&zs_zpool_driver);
+#endif
 	return 0;
-fail:
-	zs_exit();
-	return notifier_to_errno(ret);
 }
 
 static unsigned int get_maxobj_per_zspage(int size, int pages_per_zspage)