sched: fix ideal_runtime calculations for reniced tasks

fix ideal_runtime:

  - do not scale it using niced_granularity()
    it is against sum_exec_delta, so its wall-time, not fair-time.

  - move the whole check into __check_preempt_curr_fair()
    so that wakeup preemption can also benefit from the new logic.

this also results in code size reduction:

   text    data     bss     dec     hex filename
  13391     228    1204   14823    39e7 sched.o.before
  13369     228    1204   14801    39d1 sched.o.after

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