lowmemorykiller: use swapfree instead of swaptotal for better memory reclaim

Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Carlos Jesús <xxx.reptar.rawrr.xxx@gmail.com>
 lowmemorykiller: also count zram swapfile usage

however, use only 1/2 zram stats