lowmemorykiller: enable oom reaper on a lmk killed process by default

Enable OOM reaper on a LMK killed process by default. This reaping can
help us in early release of pinned memory, like cma pages, to the
system.

Change-Id: Ic0626e621d13cb77317f4fc058a706a802ee3163
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>