do nbuf constraint based on KERNEND/KERNBASE to avoid integer overflow from new giant sparc64 kvm; ok kettenis, tested by pooleb
do nbuf constraint based on KERNEND/KERNBASE to avoid integer overflow from new giant sparc64 kvm; ok kettenis, tested by pooleb

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars