sched: revert revert of: fair-group: SMP-nice for group scheduling

Try again..

Initial commit: 18d95a28
Revert: 6363ca57



Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>