kswapd: reduce time spent in congestion_wait

By reducing the amount of time kswapd spends in
congestion_wait kswapd is able to free up more
memory in the background and therefore  reduce
the amount  of time spent in direct reclaim.

Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Greg Grisco <ggrisco@codeaurora.org>