The driver core can register and cleanup sysfs groups already.
Make use of that functionality to simplify the error handling and
cleanup.

Also avoid a UAF race during unregistering where the sysctl attributes
were usable after the info struct was freed.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Helge Deller <deller@gmx.de>