kobject: fix up kobject_set_name to use kvasprintf

Kay pointed out that kobject_set_name was being very stupid, doing two
allocations for every call, when it should just be using the kernel
function kvasprintf() instead.

This change adds the internal kobject_set_name_vargs() function, which
other follow-on patches will be using.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>