sysfs: make kobj point to sysfs_dirent instead of dentry

As kobj sysfs dentries and inodes are gonna be made reclaimable,
dentry can't be used as naming token for sysfs file/directory, replace
kobj->dentry with kobj->sd.  The only external interface change is
shadow directory handling.  All other changes are contained in kobj
and sysfs.

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