Don't prefer processes with negative oom_score_adj

[hakavlad]

For example, systemd-udevd has oom_score_adj=-1000.
If I set --prefer systemd-udevd, it will get badness=300.

фев 11 21:05:09 PC earlyoom[15282]: pid   363: badness 300 vm_rss   3636 systemd-udevd
фев 11 21:05:09 PC earlyoom[15282]:     ^ new victim (higher badness)

I think it's not OK.
We can assume that this is a critical process if it has such a low oom_score_adj value, and killing it could destroy the system.

My suggestion:
Do not increase badness of processes with negative oom_score_adj. At least don't prefer processes with oom_score_adj=-1000.

[rfjakob]

Hmm. We have to calculate oom_score_before_adjustment + 300 and use that. However, I don't see a way to get oom_score_before_adjustment:

/proc/10269$ grep . comm oom_*
comm:systemd-udevd
oom_adj:-17
oom_score:0
oom_score_adj:-1000

[rfjakob]

I think if a user wants to have systemd-udevd killed, we will not prevent him from doing that, but I have documented the bug as 953cf95 .