Fix a problem noticed by Ed Tomlinson: under shifting workloads the
shrink_zone() logic will refill the inactive load too slowly.

Bale out of the zone scan when we've reclaimed enough pages.  Fixes a
rarely-occurring problem wherein refill_inactive_zone() ends up
shuffling 100,000 pages and generally goes silly.

This needs to be revisited - we should go on and rebalance the lower
zones even if we reclaimed enough pages from highmem.