64-bit file sizes. This step simply addresses the remaining overflows,
and does attempt to optimise performance. The details are:
o Use a 64-bit type for the vm_object `size' and the size argument
to vm_object_allocate().
o Use the correct type for index variables in dev_pager_getpages(),
vm_object_page_clean() and vm_object_page_remove().
o Avoid an overflow in the i386 pmap_object_init_pt().
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