Make log writing append

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/log.c b/log.c
index 611aa9f..01e4ad0 100644
--- a/log.c
+++ b/log.c
@@ -388,7 +388,7 @@
 	FILE *f;
 	unsigned int i;
 
-	f = fopen(td->o.write_iolog_file, "w+");
+	f = fopen(td->o.write_iolog_file, "a");
 	if (!f) {
 		perror("fopen write iolog");
 		return 1;
@@ -455,7 +455,7 @@
 	unsigned int i;
 	FILE *f;
 
-	f = fopen(name, "w");
+	f = fopen(name, "a");
 	if (!f) {
 		perror("fopen log");
 		return;