sched: Make sched_class::get_rr_interval() optional

Not all classes implement (or can implement) a useful get_rr_interval()
function, default to a 0 time-slice for them.

This fixes a crash reported by Tommi Rantala.
Reported-by: Tommi Rantala <tt.rantala@gmail.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140127105413.GC11314@laptop.programming.kicks-ass.netSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -4324,6 +4324,8 @@ SYSCALL_DEFINE2(sched_rr_get_interval, pid_t, pid,
 		goto out_unlock;
 
 	rq = task_rq_lock(p, &flags);
+	time_slice = 0;
+	if (p->sched_class->get_rr_interval)
 		time_slice = p->sched_class->get_rr_interval(rq, p);
 	task_rq_unlock(rq, p, &flags);