rmap: nonlinear truncation

[Hugh Dickins]

If you truncate the file beneath a nonlinear vma, your anon_vma
warns in __remove_from_page_cache then BUGs in page_referenced.
My anonmm leaves the pages unfreeable until the vma is unmapped.
pte_chains treats them as anonymous and can swap them out.  None
of us is right (allocating swap violates strict commit accounting).
I think we need to fix nonlinear truncation properly at last, I'm
looking at Daniel's invalidate_mmap_range patch from a month ago,
to see if we can springboard off that.

Hugh

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