gfio: clear graphs for repeated runs on same connection

Otherwise we get overlapping data.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/graph.h b/graph.h
index 42f781f..078e50c 100644
--- a/graph.h
+++ b/graph.h
@@ -93,5 +93,7 @@
 extern void graph_set_base_offset(struct graph *g, unsigned int base_offset);
 extern void graph_set_graph_all_zeroes(struct graph *g, unsigned int set);
 
+extern void graph_clear_values(struct graph *g);
+
 #endif