mm: oom: enable the sysctl_reap_mem_on_sigkill by default

Enable the reaping of memory on a sigkill by default. This helps in
faster releasing of memory pages to buddy.

Change-Id: I90a508e6d16099527991674f6b4efdc55f415c59
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>