block: NULL dereference on error path in __blkdev_get()

"disk" is always NULL when we goto out. There was a check for this
before, but it was removed in 69e02c59a7d9 "block: Don't check events
while open is in progress".

Signed-off-by: Dan Carpenter <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Signed-off-by: Jens Axboe <axboe@carl>

fs/block_dev.c

···
  out_unlock_bdev:
 	mutex_unlock(&bdev->bd_mutex);
 	disk_unblock_events(disk);
- out:
 	module_put(disk->fops->owner);
 	put_disk(disk);
+ out:
 	bdput(bdev);
 
 	return ret;