raid5: fix to detect failure of register_shrinker

register_shrinker can fail after commit 1d3d4437 ("vmscan: per-node
deferred work"), we should detect the failure of it, otherwise we may
fail to register shrinker after raid5 configuration was setup successfully.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid5.c

@@ -6670,7 +6670,12 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	conf->shrinker.count_objects = raid5_cache_count;
 	conf->shrinker.batch = 128;
 	conf->shrinker.flags = 0;
-	register_shrinker(&conf->shrinker);
+	if (register_shrinker(&conf->shrinker)) {
+		printk(KERN_ERR
+		       "md/raid:%s: couldn't register shrinker.\n",
+		       mdname(mddev));
+		goto abort;
+	}
 
 	sprintf(pers_name, "raid%d", mddev->new_level);
 	conf->thread = md_register_thread(raid5d, mddev, pers_name);