sched: refine negative nice level granularity

refine the granularity of negative nice level tasks: let them
reschedule more often to offset the effect of them consuming
their wait_runtime proportionately slower. (This makes nice-0
task scheduling smoother in the presence of negatively
reniced tasks.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --cc kernel/sched_fair.c
Simple merge