x86: cap iomem_resource to addressable physical memory

iomem_resource is by default initialized to -1, which means 64 bits of
physical address space if 64-bit resources are enabled.  However, x86
CPUs cannot address 64 bits of physical address space.  Thus, we want
to cap the physical address space to what the union of all CPU can
actually address.

Without this patch, we may end up assigning inaccessible values to
uninitialized 64-bit PCI memory resources.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Martin Mares <mj@ucw.cz>
Cc: stable@kernel.org