tracing: tprobe-events: Fix to register tracepoint correctly

Since __tracepoint_user_init() calls tracepoint_user_register() without
initializing tuser->tpoint with given tracpoint, it does not register
tracepoint stub function as callback correctly, and tprobe does not work.

Initializing tuser->tpoint correctly before tracepoint_user_register()
so that it sets up tracepoint callback.

I confirmed below example works fine again.

echo "t sched_switch preempt prev_pid=prev->pid next_pid=next->pid" > /sys/kernel/tracing/dynamic_events
echo 1 > /sys/kernel/tracing/events/tracepoints/sched_switch/enable
cat /sys/kernel/tracing/trace_pipe

Link: https://lore.kernel.org/all/176244793514.155515.6466348656998627773.stgit@devnote2/

Fixes: 2867495 ("tracing: tprobe-events: Register tracepoint when enable tprobe event")
Reported-by: Beau Belgrave <[email protected]>
Cc: [email protected]
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
Tested-by: Beau Belgrave <[email protected]>
Reviewed-by: Beau Belgrave <[email protected]>

Author: mhiramat

kernel/trace/trace_fprobe.c

@@ -106,13 +106,14 @@ static struct tracepoint_user *__tracepoint_user_init(const char *name, struct t
 	if (!tuser->name)
 		return NULL;
 
+	/* Register tracepoint if it is loaded. */
 	if (tpoint) {
+		tuser->tpoint = tpoint;
 		ret = tracepoint_user_register(tuser);
 		if (ret)
 			return ERR_PTR(ret);
 	}
 
-	tuser->tpoint = tpoint;
 	tuser->refcount = 1;
 	INIT_LIST_HEAD(&tuser->list);
 	list_add(&tuser->list, &tracepoint_user_list);