mm/page-writeback - highmem_is_dirtyable option (replacesdirty_highmem patch)

Well, it was quick enough to just do - here's the patch. I've also updated the documentation a bit to clarify the intention and
the reasons why you might want to use it (based in part on the comments to the original change that made highmem uncountable for dirtyness purposes)

Tested and applied against 2.6.23.9 (our build script makes Debian packages from a clean unpack of kernel major plus patch minor plus svn checkout of out quilt series and apply regardless, so it was just as easy to bump the version number while I was at it).

Builds, boots, passes a quick run of the test program I used last time around.

Bron.