Add ->open to struct fio_file

Don't use ->fd == -1 to check for the file being open or not,
an io engine could be non-fd based.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 641ecda..ceeb6e6 100644
--- a/fio.h
+++ b/fio.h
@@ -244,6 +244,7 @@
 	unsigned int last_free_lookup;
 
 	unsigned int unlink;
+	unsigned int open;
 };
 
 /*