mm: oom_kill: Support further prioritization of OOM kills

When the OOM killer selects a task to kill it considers all tasks
with an oom_score_adj value > OOM_SCORE_ADJ_MIN. This doesn't work
well when the OS has 'high priority' tasks which should only be
considered for killing after all other 'lower priority' tasks
have first been killed.

Add support for two new configs, PRIORITIZE_OOM_TASKS and
OOM_TASK_PRIORITY_ADJ_LIMIT, which allows the system to be configured
so that the OOM killer will first try to kill tasks which have an
oom_score_adj greater than a certain value.

Change-Id: Ic613138ae147427fd69b447ca5b674619ca625b2
Signed-off-by: Liam Mark <lmark@codeaurora.org>