mm, oom: normalize oom scores to oom_score_adj scale only for userspace

The oom_score_adj scale ranges from -1000 to 1000 and represents the
proportion of memory available to the process at allocation time.  This
means an oom_score_adj value of 300, for example, will bias a process as
though it was using an extra 30.0% of available memory and a value of
-350 will discount 35.0% of available memory from its usage.

The oom killer badness heuristic also uses this scale to report the oom
score for each eligible process in determining the "best" process to
kill.  Thus, it can only differentiate each process's memory usage by
0.1% of system RAM.

On large systems, this can end up being a large amount of memory: 256MB
on 256GB systems, for example.

This can be fixed by having the badness heuristic to use the actual
memory usage in scoring threads and then normalizing it to the
oom_score_adj scale for userspace.  This results in better comparison
between eligible threads for kill and no change from the userspace
perspective.

Suggested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Tested-by: Dave Jones <davej@redhat.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>