x86/crash: fix crash_setup_memmap_entries() out-of-bounds access

[Mike Galbraith]

On Fri, 2021-04-16 at 19:07 +0800, Dave Young wrote:
>
> > We're excluding two ranges, allocate the scratch space we need to do that.
>
> I think 1 range should be fine, have you tested 1?

Have now, and vzalloc(struct_size(cmem, ranges, 1)) worked just fine.

	-Mike