maps3: pagemap monitoring v3

This patchset is version 3 of my /proc/pid/pagemaps code.

Rather than submit about 30 incremental patches atop an existing 20 or
so where many of the intermediate states are broken and get undone
anyway, I've respun this as a much smaller set of 11 patches.

Changes in this series:

- headers gone again (as recommended by Dave Hansen and Alan Cox)
- 64-bit entries (as per discussion with Andi Kleen)
- swap pte information exported (from Dave Hansen)
- page walker callback for holes (from Dave Hansen)
- direct put_user I/O (as suggested by Rusty Russell)
- split kpagemap into kpagecount and kpageflags

I've dropped one cleanup patch from Rusty from the current series,
mmaps2-vma-out-of-mem_size_stats.patch, which I didn't find to be an
improvement.

Andrew, please replace the current maps2* patches with this set at
your convenience. I've included the above change list in the relevant
patches.