Dump actual filename in failure to open

Helpful for debugging.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 2b4b3c6..ab8f1d9 100644
--- a/fio.h
+++ b/fio.h
@@ -411,12 +411,14 @@
 	unsigned int cpucycle;
 };
 
+#define FIO_VERROR_SIZE	128
+
 /*
  * This describes a single thread/process executing a fio job.
  */
 struct thread_data {
 	struct thread_options o;
-	char verror[128];
+	char verror[FIO_VERROR_SIZE];
 	pthread_t thread;
 	int thread_number;
 	int groupid;