rcu: Shrink TINY_RCU by moving exit_rcu()

Now that TINY_PREEMPT_RCU is no more, exit_rcu() is always an empty
function.  But if TINY_RCU is going to have an empty function, it should
be in include/linux/rcutiny.h, where it does not bloat the kernel.
This commit therefore moves exit_rcu() out of kernel/rcupdate.c to
kernel/rcutree_plugin.h, and places a static inline empty function in
include/linux/rcutiny.h in order to shrink TINY_RCU a bit.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 51230b6..e31005e 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -119,6 +119,10 @@
 {
 }
 
+static inline void exit_rcu(void)
+{
+}
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 extern int rcu_scheduler_active __read_mostly;
 extern void rcu_scheduler_starting(void);