[PATCH] vmscan: drive everything via nr_to_scan

[PATCH] vmscan: drive everything via nr_to_scan

Page reclaim is currently a bit schitzo: sometimes we say "go and scan this
many pages and tell me how many pages were freed" and at other times we say
"go and scan this many pages, but stop if you freed this many".

It makes the logic harder to control and to understand.  This patch coverts
everything into the "go and scan this many pages and tell me how many pages
were freed" model.

It doesn't seem to affect performance much either way.