Previously we assumed in many places that all pages between a VMA's
vma_start and vma_end belonged to that VMA.  For radix, the radix tree
is now the source of truth about which pages a VMA maps.  This will
let us replace subranges of a VMA without having to split the VMA into
two ranges (which would violate the commutativity rule; e.g. for
disjoint unmaps within the same VMA).