sched: let +nice tasks have smaller impact

sched: let +nice tasks have smaller impact

Michel Dänzr has bisected an interactivity problem with
plus-reniced tasks back to this commit:

 38a989fe1a251334cb53694c86406a02734bce07 is first bad commit
 commit 38a989fe1a251334cb53694c86406a02734bce07
 Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
 Date:   Mon Oct 15 17:00:14 2007 +0200

 sched: another wakeup_granularity fix

      unit mis-match: wakeup_gran was used against a vruntime

fix this by assymetrically scaling the vtime of positive reniced
tasks.

Bisected-by: Michel Dänzer <michel@tungstengraphics.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>