With the shadow directories gone, sysfs_rename_dir() can be simplified.

* parent doesn't need to be grabbed separately.  Just access
  old_dentry->d_parent.

* parent sd can never change.  Remove code to move under the new
  parent.

* Massage comments a bit.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Acked-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>