oom: make oom_score to per-process value

oom: make oom_score to per-process value

oom-killer kills a process, not task.  Then oom_score should be calculated
as per-process too.  it makes consistency more and makes speed up
select_bad_process().

Signed-off-by: KOSAKI Motohiro <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>