block: move holder_dir from disk to part0

Move disk->holder_dir to part0->holder_dir.  Kill now mostly
superflous bdev_get_holder().

While at it, kill superflous kobject_get/put() around holder_dir,
slave_dir and cmd_filter creation and collapse
disk_sysfs_add_subdirs() into register_disk().  These serve no purpose
but obfuscating the code.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/block_dev.c b/fs/block_dev.c
index c982a91..57d5726 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -548,14 +548,6 @@
 		return kobject_get(&disk_to_dev(bdev->bd_disk)->kobj);
 }
 
-static struct kobject *bdev_get_holder(struct block_device *bdev)
-{
-	if (bdev->bd_contains != bdev)
-		return kobject_get(bdev->bd_part->holder_dir);
-	else
-		return kobject_get(bdev->bd_disk->holder_dir);
-}
-
 static int add_symlink(struct kobject *from, struct kobject *to)
 {
 	if (!from || !to)
@@ -608,7 +600,7 @@
 	if (!bo->sdev)
 		goto fail_put_hdev;
 
-	bo->hdir = bdev_get_holder(bdev);
+	bo->hdir = kobject_get(bdev->bd_part->holder_dir);
 	if (!bo->hdir)
 		goto fail_put_sdev;