sched: sched_setscheduler() fix

Fix a problem in the 'sched-group' patch for !CONFIG_FAIR_GROUP_SCHED.

description:

sched_setscheduler()
{
...
if (task_running()) p->sched_class->put_prev_entity();

[ this one sets up cfs_rq->curr to NULL ]

...

if (task_running) p->sched_class->set_curr_task();

[ and this one is a _NOP_ (empty) for !CONFIG_FAIR_GROUP_SCHED ]

As a result, the task continues to run with cfs_rq->curr == NULL... no 
crashes (due to checks for !NULL in place) but e.g. update_curr() 
effectively becomes a NOP... i.e. runtime statistics for this task is 
not accounted untill it's rescheduled anew.

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>