tracing: Add WARN_ON_ONCE when returned value is negative

ret is assigned return value of event_hist_trigger_func, but the value
is unused. It is better to warn when returned value is negative,
rather than just ignoring it.

Link: https://lkml.kernel.org/r/20210529061423.GA103954@hyeyooSigned-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -5232,6 +5232,7 @@ static void unregister_field_var_hists(struct hist_trigger_data *hist_data)
 		cmd = hist_data->field_var_hists[i]->cmd;
 		ret = event_hist_trigger_func(&trigger_hist_cmd, file,
 					      "!hist", "hist", cmd);
+		WARN_ON_ONCE(ret < 0);
 	}
 }