Strengthen the `incremental min' logic in the page allocator.

Currently it is allowing the allocation to succeed if the zone has
free_pages >= pages_high.

This was to avoid a lockup corner case in which all the zones were at
pages_high so reclaim wasn't doing anything, but the incremental min
refused to take pages from those zones anyway.

But we want the incremental min zone protection to work.  So:

- Only allow the allocator to dip below the incremental min if he
  cannot run direct reclaim.

- Change the page reclaim code so that on the direct reclaim path,
  the caller can free pages beyond ->pages_high.  So if the incremental
  min test fails, the caller will go and free some more memory.

  Eventually, the caller will have freed enough memory for the
  incremental min test to pass against one of the zones.