bcache: cleaned up error handling around register_cache()

Fix null pointer dereference by changing register_cache() to return an int
instead of being void.  This allows it to return -ENOMEM or -ENODEV and
enables upper layers to handle the OOM case without NULL pointer issues.

See this thread:
  http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3521

Fixes this error:
  gargamel:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register

  bcache: register_cache() error opening sdh2: cannot allocate memory
  BUG: unable to handle kernel NULL pointer dereference at 00000000000009b8
  IP: [<ffffffffc05a7e8d>] cache_set_flush+0x102/0x15c [bcache]
  PGD 120dff067 PUD 1119a3067 PMD 0
  Oops: 0000 [#1] SMP
  Modules linked in: veth ip6table_filter ip6_tables
  (...)
  CPU: 4 PID: 3371 Comm: kworker/4:3 Not tainted 4.4.2-amd64-i915-volpreempt-20160213bc1 #3
  Hardware name: System manufacturer System Product Name/P8H67-M PRO, BIOS 3904 04/27/2013
  Workqueue: events cache_set_flush [bcache]
  task: ffff88020d5dc280 ti: ffff88020b6f8000 task.ti: ffff88020b6f8000
  RIP: 0010:[<ffffffffc05a7e8d>]  [<ffffffffc05a7e8d>] cache_set_flush+0x102/0x15c [bcache]
Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net>
Tested-by: Marc MERLIN <marc@merlins.org>
Cc: <stable@vger.kernel.org>

drivers/md/bcache/super.c

@@ -1835,11 +1835,12 @@ static int cache_alloc(struct cache_sb *sb, struct cache *ca)
 	return 0;
 }
 
-static void register_cache(struct cache_sb *sb, struct page *sb_page,
+static int register_cache(struct cache_sb *sb, struct page *sb_page,
 				struct block_device *bdev, struct cache *ca)
 {
 	char name[BDEVNAME_SIZE];
-	const char *err = "cannot allocate memory";
+	const char *err = NULL;
+	int ret = 0;
 
 	memcpy(&ca->sb, sb, sizeof(struct cache_sb));
 	ca->bdev = bdev;
@@ -1854,27 +1855,35 @@ static void register_cache(struct cache_sb *sb, struct page *sb_page,
 	if (blk_queue_discard(bdev_get_queue(ca->bdev)))
 		ca->discard = CACHE_DISCARD(&ca->sb);
 
-	if (cache_alloc(sb, ca) != 0)
+	ret = cache_alloc(sb, ca);
+	if (ret != 0)
 		goto err;
 
-	err = "error creating kobject";
-	if (kobject_add(&ca->kobj, &part_to_dev(bdev->bd_part)->kobj, "bcache"))
-		goto err;
+	if (kobject_add(&ca->kobj, &part_to_dev(bdev->bd_part)->kobj, "bcache")) {
+		err = "error calling kobject_add";
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	mutex_lock(&bch_register_lock);
 	err = register_cache_set(ca);
 	mutex_unlock(&bch_register_lock);
 
-	if (err)
-		goto err;
+	if (err) {
+		ret = -ENODEV;
+		goto out;
+	}
 
 	pr_info("registered cache device %s", bdevname(bdev, name));
+
 out:
 	kobject_put(&ca->kobj);
-	return;
+
 err:
+	if (err)
 		pr_notice("error opening %s: %s", bdevname(bdev, name), err);
-	goto out;
+
+	return ret;
 }
 
 /* Global interfaces/init */
@@ -1972,7 +1981,8 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 		if (!ca)
 			goto err_close;
 
-		register_cache(sb, sb_page, bdev, ca);
+		if (register_cache(sb, sb_page, bdev, ca) != 0)
+			goto err_close;
 	}
 out:
 	if (sb_page)