RFC: x86: cap iomem_resource to addressable physical memory

[H. Peter Anvin]

This is a multi-part message in MIME format.x86 cannot generate full 64-bit addresses; this patch clamps iomem
addresses to the accessible range.

I wanted to post it for review before committing it, however; comments
would be appreciated, especially of the kind "this is done too early/too
late/in the wrong place/incorrectly".

	-hpa