lowmemorykiller: Support asynchronous compaction in lowmemorykiller.

Signed-off-by: Cerebro <zarboz@gmail.com>

Conflicts:
drivers/staging/android/lowmemorykiller.c

idea is taken here -
https://github.com/zarboz/M7wlv_KK_kernel/commit/7be90ab9aa70c0415be60743ac6df6e5a98d6b79