ia32 and others can determine a page's hugeness by inspecting the pmd's value
directly.  No need to perform a VMA lookup against the user's virtual
address.

This patch ifdef's away the VMA-based implementation of
hugepage-aware-follow_page for ia32 and replaces it with a pmd-based
implementation.

The intent is that architectures will implement one or the other.  So the architecture either:

1: Implements hugepage_vma()/follow_huge_addr(), and stubs out
   pmd_huge()/follow_huge_pmd() or

2: Implements pmd_huge()/follow_huge_pmd(), and stubs out
   hugepage_vma()/follow_huge_addr()