kswapd: Introduce congestion_wait_kswapd

congestion_wait for kswapd is similar to congestion_wait, the vital
difference is that congestion_wait_kswapd uses schedule_timeout
instead of io_schedule_timeout (which can cause IO wait conditions,
due to nr_iowait usage).

Derived from: https://github.com/oppo-source/Find5-Kernel-Source-4.2/commit/c7b44a43a970b699ecd68655bd6f83a8fbb3d74d
Original author: huanggd @oppo-source

Change-Id: I08bbbdd49c8764a3180c0d7a9197d2b9c8112d81
Signed-off-by: Luis Cruz <ljc2491@gmail.com>