lowmemorykiller: use oom reaper to free pages of task killed by lmk

Free the pages parallely for a task that is LMK killed using the
oom_reaper. This freeing of pages will help to give the pages to
buddy system well advance there by we may achieve less number of
killings by LMK.

Change-Id: I5e1ed183437ab243f12cbbf3ae10d9ca5211fc06
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>