[PATCH] hot-n-cold pages: use cold pages for readahead

[PATCH] hot-n-cold pages: use cold pages for readahead

It is usually the case that pagecache reads use busmastering hardware
to transfer the data into pagecache.  This invalidates the CPU cache of
the pagecache pages.

So use cache-cold pages for pagecache reads.  To avoid wasting
cache-hot pages.