ftrace: fix mutex unlock in trace output

If the trace output changes on reading the trace files, there is a chance
that the start function will return NULL. If the start function of a sequence
returns NULL the stop equivalent is not called. In this case, all locks
that are taken must be released even if they are released in the stop function.

This patch fixes a case that a mutex was not released on return of NULL
in the start sequence function.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/trace/trace.c

@@ -964,8 +964,10 @@ static void *s_start(struct seq_file *m, loff_t *pos)
 
 	mutex_lock(&trace_types_lock);
 
-	if (!current_trace || current_trace != iter->trace)
+	if (!current_trace || current_trace != iter->trace) {
+		mutex_unlock(&trace_types_lock);
 		return NULL;
+	}
 
 	atomic_inc(&trace_record_cmdline_disabled);