kref: add kref_set()

This adds kref_set() to the kref api for future use by people who really
know what they are doing with krefs...

From: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>