lowmemorykiller: avoid selecting task reaped by oom_reaper

With oom_reaper enabled for lowmemorykiller, even when the oom_reaper reaps
the task, the task struct will have LMK_WAITING flag set.  As a side
effect, during lowmem_deathpending_timeout period, lowmemorykiller will not
select any task while any task has LMK_WAITING flag set.

Skip tasks from lmk selection if they are reaped by oom_reaper.

Change-Id: I7edb02135df7e54d9364b971caf25ab6ace9a027
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>