mm/oom_kill: Online memory before running oom-killer

Attempt to online memory before invoking the oom-killer
to alleviate the memory pressure in the system as quickly
as possible, without sacrificing performance/user-experience
by invoking the oom-killer.

Change-Id: I0cf3c50fcd022cd282731bccb54bfbad45878bb1
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>