Add early-boot-safety check to cond_resched()

Just to be safe, we should not trigger a conditional reschedule during
the early boot sequence.  We've historically done some questionable
early on, and the safety warnings in __might_sleep() are generally
turned off during that period, so there might be problems lurking.

This affects CONFIG_PREEMPT_VOLUNTARY, which takes over might_sleep() to
cause a voluntary conditional reschedule.
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4028,6 +4028,8 @@ static inline void __cond_resched(void)
 	 */
 	if (unlikely(preempt_count()))
 		return;
+	if (unlikely(system_state != SYSTEM_RUNNING))
+		return;
 	do {
 		add_preempt_count(PREEMPT_ACTIVE);
 		schedule();