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>
Signed-off-by: Paul Reioux <reioux@gmail.com>

Conflicts:
	mm/vmscan.c

Original commit: https://github.com/UpInTheAir/SM-N920/commit/b93c812a30f8e55c8cdb9fddd9ea953b3818b423