rcu: Define rcu_all_qs() only in !PREEMPT builds

Now that rcu_all_qs() is used only in !PREEMPT builds, move it to
tree_plugin.h so that it is defined only in those builds.  This in
turn means that rcu_momentary_dyntick_idle() is only used in !PREEMPT
builds, but it is simply marked __maybe_unused in order to keep it
near the rest of the dyntick-idle code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcutree.h

@@ -66,7 +66,9 @@ void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 void rcu_end_inkernel_boot(void);
 bool rcu_is_watching(void);
+#ifndef CONFIG_PREEMPT
 void rcu_all_qs(void);
+#endif
 
 /* RCUtree hotplug events */
 int rcutree_prepare_cpu(unsigned int cpu);