ftrace: inject markers via trace_marker file

Allow a user to inject a marker (TRACE_PRINT entry) into the trace ring
buffer. The related file operations are derived from code by Frédéric
Weisbecker <fweisbec@gmail.com>.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.h

@@ -124,6 +124,10 @@ struct trace_entry {
 };
 
 #define TRACE_ENTRY_SIZE	sizeof(struct trace_entry)
+#define TRACE_BUF_SIZE		1024
+#define TRACE_PRINT_BUF_SIZE \
+	(sizeof(struct trace_field) - offsetof(struct trace_field, print.buf))
+#define TRACE_CONT_BUF_SIZE	sizeof(struct trace_field)
 
 /*
  * The CPU trace array - it consists of thousands of trace entries