Currently kswapd walks across all zones in dma->normal->highmem order,
performing proportional scanning until all zones are OK.  This means that
pressure against ZONE_NORMAL causes unnecessary reclaim of ZONE_HIGHMEM.

To fix that up we change kswapd so that it walks the zones in the
high->normal->dma direction, skipping zones which are OK.  Once it encounters
a zone which needs some reclaim kswapd will perform proportional scanning
against that zone as well as all the succeeding lower zones.

We scan the lower zones even if they have sufficient free pages.  This is
because

a) the lower zone may be above pages_high, but because of the incremental
   min, the lower zone may still not be eligible for allocations.  That's bad
   because cache in that lower zone will then not be scanned at the correct
   rate.

b) pages in this lower zone are usable for allocations against the higher
   zone.  So we do want to san all the relevant zones at an equal rate.