swap: trigger early LMK when swap space is not enough.

swap: trigger early LMK when swap space is not enough.

If swap device is almost full and there is no improvement to compact zram,
it means that there are many background processes or some process makes memory leak.
It will sometime make a sluggish or hangup issue.
So, it requires to trigger proactive oom killer.
Especially in case of zram, it is necessary to check memory usage of swap cgroup.
If it is enough, trigger reclaiming memory about background applications.
Otherwise, find mallicious process in all memory cgroups.

Change-Id: I14e97f7e1b58545de221ff5cf1a674153d9e98a5
Signed-off-by: ByungSoo Kim <bs1770.kim@samsung.com>
Signed-off-by: Paweł Szewczyk <p.szewczyk@samsung.com>