shrink_slab() wants to calculate

	nr_scanned_pages * seeks_per_object * entries_in_slab /
		nr_lru_pages

entries_in_slab and nr_lru_pages can vary a lot.  There is a potential
for 32-bit overflows.

I spent ages trying to avoid corner cases which cause a significant
lack of precision while preserving some clarity.  Gave up and used
do_div().  The code is called rarely - at most once per 128 kbytes of
reclaim.

The patch adds a tweak to balance_pgdat() to reduce the call rate to
shrink_slab() in the case where the zone is just a little bit below
pages_high.

Also increase SHRINK_BATCH.  The things we're shrinking are typically a
few hundred bytes, and a batchcount of 128 gives us a minimum of ten
pages or so per shrinking callout.