kobject: fix memory leak in kobject_set_name_vargs

If the call to kvasprintf fails then the old name of the object will be leaked,
this patch fixes the bug by restoring the old name before returning ENOMEM.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>