tracing: Clean up alloc_synth_event()

alloc_synth_event() currently has the following code to initialize the
event fields and dynamic_fields:

	for (i = 0, j = 0; i < n_fields; i++) {
		event->fields[i] = fields[i];

		if (fields[i]->is_dynamic) {
			event->dynamic_fields[j] = fields[i];
			event->dynamic_fields[j]->field_pos = i;
			event->dynamic_fields[j++] = fields[i];
			event->n_dynamic_fields++;
		}
	}

1) It would make more sense to have all fields keep track of their
   field_pos.

2) event->dynmaic_fields[j] is assigned twice for no reason.

3) We can move updating event->n_dynamic_fields outside the loop, and just
   assign it to j.

This combination makes the code much cleaner.

Link: https://lkml.kernel.org/r/20210721195341.29bb0f77@oasis.local.homeSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_synth.c

@@ -893,15 +893,13 @@ static struct synth_event *alloc_synth_event(const char *name, int n_fields,
 	dyn_event_init(&event->devent, &synth_event_ops);
 
 	for (i = 0, j = 0; i < n_fields; i++) {
+		fields[i]->field_pos = i;
 		event->fields[i] = fields[i];
 
-		if (fields[i]->is_dynamic) {
-			event->dynamic_fields[j] = fields[i];
-			event->dynamic_fields[j]->field_pos = i;
+		if (fields[i]->is_dynamic)
 			event->dynamic_fields[j++] = fields[i];
-			event->n_dynamic_fields++;
-		}
 	}
+	event->n_dynamic_fields = j;
 	event->n_fields = n_fields;
  out:
 	return event;