Add option for refilling IO buffers on each submit

If the device looks at whether the data changed, then this can
make a difference.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 47b6d48..25d05bd 100644
--- a/fio.h
+++ b/fio.h
@@ -478,6 +478,7 @@
 	unsigned int group_reporting;
 	unsigned int fadvise_hint;
 	unsigned int zero_buffers;
+	unsigned int refill_buffers;
 	unsigned int time_based;
 
 	char *read_iolog_file;
@@ -907,6 +908,7 @@
 extern void io_u_init_timeout(void);
 extern void io_u_set_timeout(struct thread_data *);
 extern void io_u_mark_depth(struct thread_data *, unsigned int);
+extern void io_u_fill_buffer(struct thread_data *td, struct io_u *, unsigned int);
 
 /*
  * io engine entry points