tracepoints: Use WARN() and not WARN_ON() for warnings

There are two WARN_ON*() warnings in tracepoint.h that deal with RCU
usage. But when they trigger, especially from using a TRACE_EVENT()
macro, the information is not very helpful and is confusing:

 ------------[ cut here ]------------
 WARNING: CPU: 0 PID: 0 at include/trace/events/lock.h:24 lock_acquire+0x2b2/0x2d0

Where the above warning takes you to:

 TRACE_EVENT(lock_acquire,  <<<--- line 24 in lock.h

	TP_PROTO(struct lockdep_map *lock, unsigned int subclass,
		int trylock, int read, int check,
		struct lockdep_map *next_lock, unsigned long ip),
	[..]

Change the WARN_ON_ONCE() to WARN_ONCE() and add a string that allows
someone to search for exactly where the bug happened.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Reported-by: Borislav Petkov <[email protected]>
Tested-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

include/linux/tracepoint.h

@@ -199,7 +199,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		if (!(cond))						\
 			return;						\
 									\
-		if (WARN_ON_ONCE(RCUIDLE_COND(rcuidle)))		\
+		if (WARN_ONCE(RCUIDLE_COND(rcuidle),			\
+			      "Bad RCU usage for tracepoint"))		\
 			return;						\
 									\
 		/* keep srcu and sched-rcu usage consistent */		\
@@ -259,7 +260,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 				TP_ARGS(args),				\
 				TP_CONDITION(cond), 0);			\
 		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
-			WARN_ON_ONCE(!rcu_is_watching());		\
+			WARN_ONCE(!rcu_is_watching(),			\
+				  "RCU not watching for tracepoint");	\
 		}							\
 	}								\
 	__DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),		\