lmk: fix for using implement task's adj rbtree

Based on the current LMK implementation, LMK has to
scan all processes to select the correct task to kill
during low memory.
The basic idea for the optimization is to : queue all
tasks with oom_score_adj priority, and then LMK just
selects the proper task from the queue(rbtree) to kill.
performance improvement:
current: average time to find a task to kill : 1004us
optimized: average time to find a task to kill: 43us

Change-Id: I32504e9f2f370d58c038eea7457d95c8ed8b6b9b
Signed-off-by: Hong-Mei Li <a21834@motorola.com>
Signed-off-by: Yi-wei Zhao <gbjc64@motorola.com>
Reviewed-on: http://gerrit.mot.com/701205
Reviewed-on: http://gerrit.mot.com/715645


SLTApproved: Slta Waiver <sltawvr@motorola.com>
Tested-by: Jira Key <jirakey@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>
Reviewed-on: http://gerrit.mot.com/727655


SME-Granted: SME Approvals Granted
Reviewed-by: Sheng-Zhe Zhao <a18689@motorola.com>

Based on commit: https://bitbucket.org/arter97/android_kernel_samsung_exynos7420/commits/079d92a3311540e7d57fefcc73d68205371e579b