ftrace: Have set_graph_functions handle write with RDWR

Since reading the set_graph_functions uses seq functions, which sets the
file->private_data pointer to a seq_file descriptor. On writes the
ftrace_graph_data descriptor is set to file->private_data. But if the file
is opened for RDWR, the ftrace_graph_write() will incorrectly use the
file->private_data descriptor instead of
((struct seq_file *)file->private_data)->private pointer, and this can crash
the kernel.
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -4842,6 +4842,12 @@ ftrace_graph_write(struct file *file, const char __user *ubuf,
 	if (trace_parser_get_init(&parser, FTRACE_BUFF_MAX))
 		return -ENOMEM;
 
+	/* Read mode uses seq functions */
+	if (file->f_mode & FMODE_READ) {
+		struct seq_file *m = file->private_data;
+		fgd = m->private;
+	}
+
 	read = trace_get_user(&parser, ubuf, cnt, ppos);
 
 	if (read >= 0 && trace_parser_loaded((&parser))) {