sysfs: add sysfs_dirent->s_parent

Add sysfs_dirent->s_parent.  With this patch, each sd points to and
holds a reference to its parent.  This allows walking sysfs tree
without referencing sd->s_dentry which can go away anytime if the user
doesn't control when it's deleted.

sd->s_parent is initialized and parent is referenced in
sysfs_attach_dirent().  Reference to parent is released when the sd is
released, so as long as reference to a sd is held, s_parent can be
followed.

dentry walk in sysfs_readdir() is convereted to s_parent walk.

This will be used to reimplement symlink such that it uses only
sysfs_dirent tree.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/sysfs.h

@@ -1,5 +1,6 @@
 struct sysfs_dirent {
 	atomic_t		s_count;
+	struct sysfs_dirent	* s_parent;
 	struct list_head	s_sibling;
 	struct list_head	s_children;
 	void 			* s_element;