tracing: Add pipe_close interface

An ftrace plugin can add a pipe_open interface when the user opens
trace_pipe. But if the plugin allocates something within the pipe_open
it can not free it because there exists no pipe_close. The hook to
the trace file open has a corresponding close. The closing of the
trace_pipe file should also have a corresponding close.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2898,6 +2898,10 @@ static int tracing_release_pipe(struct inode *inode, struct file *file)
 	else
 		cpumask_clear_cpu(iter->cpu_file, tracing_reader_cpumask);
 
+
+	if (iter->trace->pipe_open)
+		iter->trace->pipe_close(iter);
+
 	mutex_unlock(&trace_types_lock);
 
 	free_cpumask_var(iter->started);

kernel/trace/trace.h

@@ -272,6 +272,7 @@ struct tracer_flags {
  * @pipe_open: called when the trace_pipe file is opened
  * @wait_pipe: override how the user waits for traces on trace_pipe
  * @close: called when the trace file is released
+ * @pipe_close: called when the trace_pipe file is released
  * @read: override the default read callback on trace_pipe
  * @splice_read: override the default splice_read callback on trace_pipe
  * @selftest: selftest to run on boot (see trace_selftest.c)
@@ -290,6 +291,7 @@ struct tracer {
 	void			(*pipe_open)(struct trace_iterator *iter);
 	void			(*wait_pipe)(struct trace_iterator *iter);
 	void			(*close)(struct trace_iterator *iter);
+	void			(*pipe_close)(struct trace_iterator *iter);
 	ssize_t			(*read)(struct trace_iterator *iter,
 					struct file *filp, char __user *ubuf,
 					size_t cnt, loff_t *ppos);