tracing: Prevent deleting instances when they are being read

Add a ref count to the trace_array structure and prevent removal
of instances that have open descriptors.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 1a456c2..f493182 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -219,6 +219,7 @@
 	struct list_head	systems;
 	struct list_head	events;
 	struct task_struct	*waiter;
+	int			ref;
 };
 
 enum {