tracing: Clear trace_state when starting trace

Clear trace_state data structure when starting trace
in __synth_event_trace_start() internal function.

Currently trace_state is initialized only in the
synth_event_trace_start() API, but the trace_state
in synth_event_trace() and synth_event_trace_array()
are on the stack without initialization.
This means those APIs will see wrong parameters and
wil skip closing process in __synth_event_trace_end()
because trace_state->disabled may be !0.

Link: http://lkml.kernel.org/r/158193315899.8868.1781259176894639952.stgit@devnote2

Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -1824,6 +1824,8 @@ __synth_event_trace_start(struct trace_event_file *file,
 	int entry_size, fields_size = 0;
 	int ret = 0;
 
+	memset(trace_state, '\0', sizeof(*trace_state));
+
 	/*
 	 * Normal event tracing doesn't get called at all unless the
 	 * ENABLED bit is set (which attaches the probe thus allowing
@@ -2063,8 +2065,6 @@ int synth_event_trace_start(struct trace_event_file *file,
 	if (!trace_state)
 		return -EINVAL;
 
-	memset(trace_state, '\0', sizeof(*trace_state));
-
 	ret = __synth_event_trace_start(file, trace_state);
 	if (ret == -ENOENT)
 		ret = 0; /* just disabled, not really an error */