tracing: add binary buffer files for use with splice

Impact: new feature

This patch creates a directory of files that correspond to the
per CPU ring buffers. These are binary files and are made to
be used with splice. This is the fastest way to extract data from
the ftrace ring buffers.

Thanks to Jiaying Zhang for pushing me to get this code fixed,
 and to Eduard - Gabriel Munteanu for his splice code that helped
 me debug my code.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace.h

@@ -217,6 +217,7 @@ enum trace_flag_type {
  */
 struct trace_array_cpu {
 	atomic_t		disabled;
+	void			*buffer_page;	/* ring buffer spare */
 
 	/* these fields get copied into max-trace: */
 	unsigned long		trace_idx;