mm: vmpressure: fix scan window after SWAP_CLUSTER_MAX increase

mm: vmpressure: fix scan window after SWAP_CLUSTER_MAX increase

mm-increase-swap_cluster_max-to-batch-tlb-flushes.patch changed
SWAP_CLUSTER_MAX from 32 pages to 256 pages, inadvertantly switching the
scan window for vmpressure detection from 2MB to 16MB.  Fix.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>