fs: support task's adj rbtree

Add (or del) a task to (or from) task's adj rbtree
when its oom_score_adj is modified.
Change-Id: I8a9490c6f30f42f6e4ed8f7fa9249f6da6177ae2
Signed-off-by: Hong-Mei Li <a21834@motorola.com>
Signed-off-by: Yi-wei Zhao <gbjc64@motorola.com>
Reviewed-on: http://gerrit.mot.com/701206


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/727657


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

Used: https://bitbucket.org/arter97/android_kernel_samsung_exynos7420/commits/6c2c82dab951a54a99dc82270b42ffeb01b6a511