Page allocator: get rid of the list of cold pages

We have repeatedly discussed if the cold pages still have a point. There is
one way to join the two lists: Use a single list and put the cold pages at the
end and the hot pages at the beginning. That way a single list can serve for
both types of allocations.

The discussion of the RFC for this and Mel's measurements indicate that
there may not be too much of a point left to having separate lists for
hot and cold pages (see http://marc.info/?t=119492914200001&r=1&w=2).

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Martin Bligh <mbligh@mbligh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>