mm: vmscan: update from lk 3.0.101

Change-Id: Icd6e87c42a2826b01b6039952f3af219d3aa3e72
mm: vmscan: If kswapd has been running too long, allow it to sleep

Under constant allocation pressure, kswapd can be in the situation where
sleeping_prematurely() will always return true even if kswapd has been
running a long time. Check if kswapd needs to be scheduled.

Signed-off-by: Mel Gorman <mgorman@suse.de>