Do not cache pages belonging to memory ranges with a `use' count.

Do not cache pages belonging to memory ranges with a `use' count.

Such pages belong to the DMA or ISA memory ranges and caching them
accelerate their exhaustion.  On amd64, at least, the kernel relies
on having low pages available at any time and cannot recover from
their exhaustion.

Should prevent livelocks reported by jsg@ and tb@ on amd64.

ok deraadt@