mm, oom: fix badness score underflow

If the privileges given to root threads (3% of allowable memory) or a
negative value of /proc/pid/oom_score_adj happen to exceed the amount of
rss of a thread, its badness score overflows as a result of commit
a7f638f999ff ("mm, oom: normalize oom scores to oom_score_adj scale only
for userspace").

Fix this by making the type signed and return 1, meaning the thread is
still eligible for kill, if the value is negative.

Reported-by: Dave Jones <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Signed-off-by: David Rientjes <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/oom_kill.c

···
 unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 			  const nodemask_t *nodemask, unsigned long totalpages)
 {
-	unsigned long points;
+	long points;
 
 	if (oom_unkillable_task(p, memcg, nodemask))
 		return 0;
···
 	 * Never return 0 for an eligible task regardless of the root bonus and
 	 * oom_score_adj (oom_score_adj can't be OOM_SCORE_ADJ_MIN here).
 	 */
-	return points ? points : 1;
+	return points > 0 ? points : 1;
 }
 
 /*