mm, oom: fix badness score underflow

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 <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Signed-off-by: David Rientjes <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>