rcu: fix build error for sysrq workaround

I got a build error in linux-next today, coming from:
http://ozlabs.org/~akpm/mmots/broken-out/sysrqrcu-suppress-rcu-stall-warnings-while-sysrq-runs.patch

I saw the problem has already been reported by other people, but couldn't find a fix. This is what I used locally to avoid the build error. Not sure if it's correct to do nothing though.

Please just ignore if the problem has already been solved independently.

Arnd

(re-sent with mailing list on Cc)

9<-----------
From: Arnd Bergmann <[email protected]>
Subject: [PATCH] rcu: fix build error for sysrq workaround

Patch "sysrq,rcu: suppress RCU stall warnings while sysrq runs" introduced two new functions, but they are unavailable if CONFIG_RCU_STALL_COMMON is not set, so we have to provide an alternative implementation, doing nothing.

index e9cc71c..26e1771 100644
@@ -247,8 +247,15 @@ void rcu_idle_enter(void);
 void rcu_idle_exit(void);
 void rcu_irq_enter(void);
 void rcu_irq_exit(void);
+
+#ifdef CONFIG_RCU_STALL_COMMON
 void rcu_sysrq_start(void);
 void rcu_sysrq_end(void);
+#else
+static inline void rcu_sysrq_start(void) { }
+static inline void rcu_sysrq_end(void) { }
+
+#endif
 
 #ifdef CONFIG_RCU_USER_QS
 void rcu_user_enter(void);