sched: Un-hide lockdep_tasklist_lock_is_held() for !LOCKDEP

Currently, variables used only within lockdep expressions are flagged as
unused, requiring that these variables' declarations be decorated with
either #ifdef or __maybe_unused.  This results in ugly code.  This commit
therefore causes the lockdep_tasklist_lock_is_held() function to be
visible even when lockdep is not enabled, thus removing the need for
these decorations.  This approach further relies on dead-code elimination
to remove any references to functions or variables that are not available
in non-lockdep kernels.
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

include/linux/sched/task.h

@@ -47,9 +47,7 @@ extern spinlock_t mmlist_lock;
 extern union thread_union init_thread_union;
 extern struct task_struct init_task;
 
-#ifdef CONFIG_PROVE_RCU
 extern int lockdep_tasklist_lock_is_held(void);
-#endif /* #ifdef CONFIG_PROVE_RCU */
 
 extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);