block: pre-initialize struct block_device in bdev_alloc_inode

bdev_evict_inode and bdev_free_inode are also called for the root inode
of bdevfs, for which bdev_alloc is never called.  Move the zeroing o
f struct block_device and the initialization of the bd_bdi field into
bdev_alloc_inode to make sure they are initialized for the root inode
as well.

Fixes: e6cb5382 ("block: initialize struct block_device in bdev_alloc")
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/block_dev.c

@@ -776,8 +776,11 @@ static struct kmem_cache * bdev_cachep __read_mostly;
 static struct inode *bdev_alloc_inode(struct super_block *sb)
 {
 	struct bdev_inode *ei = kmem_cache_alloc(bdev_cachep, GFP_KERNEL);
+
 	if (!ei)
 		return NULL;
+	memset(&ei->bdev, 0, sizeof(ei->bdev));
+	ei->bdev.bd_bdi = &noop_backing_dev_info;
 	return &ei->vfs_inode;
 }
 
@@ -871,14 +874,12 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 	mapping_set_gfp_mask(&inode->i_data, GFP_USER);
 
 	bdev = I_BDEV(inode);
-	memset(bdev, 0, sizeof(*bdev));
 	mutex_init(&bdev->bd_mutex);
 	mutex_init(&bdev->bd_fsfreeze_mutex);
 	spin_lock_init(&bdev->bd_size_lock);
 	bdev->bd_disk = disk;
 	bdev->bd_partno = partno;
 	bdev->bd_inode = inode;
-	bdev->bd_bdi = &noop_backing_dev_info;
 #ifdef CONFIG_SYSFS
 	INIT_LIST_HEAD(&bdev->bd_holder_disks);
 #endif