rcu-tasks: Move #ifdef into tasks.h

This commit pushes the #ifdef CONFIG_TASKS_RCU_GENERIC from
kernel/rcu/update.c to kernel/rcu/tasks.h in order to improve
readability as more APIs are added.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -5,6 +5,7 @@
  * Copyright (C) 2020 Paul E. McKenney
  */
 
+#ifdef CONFIG_TASKS_RCU_GENERIC
 
 ////////////////////////////////////////////////////////////////////////
 //
@@ -984,3 +985,7 @@ core_initcall(rcu_spawn_tasks_trace_kthread);
 #else /* #ifdef CONFIG_TASKS_TRACE_RCU */
 void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
 #endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
+
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+static inline void rcu_tasks_bootup_oddness(void) {}
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */

kernel/rcu/update.c

@@ -584,11 +584,7 @@ late_initcall(rcu_verify_early_boot_tests);
 void rcu_early_boot_tests(void) {}
 #endif /* CONFIG_PROVE_RCU */
 
-#ifdef CONFIG_TASKS_RCU_GENERIC
 #include "tasks.h"
-#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
-static inline void rcu_tasks_bootup_oddness(void) {}
-#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 
 #ifndef CONFIG_TINY_RCU