sched: fix vslice

vslice was missing a factor NICE_0_LOAD, as weight is in
weight*NICE_0_LOAD units.

the effect of this bug was larger initial slices and
thus latency-noisier forks.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>