Better check for not doing a file twice

Mark it as done when we close it, so we don't open the same
one again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 76d7ea4..5ca72c5 100644
--- a/fio.h
+++ b/fio.h
@@ -240,6 +240,7 @@
 	FIO_FILE_EXISTS		= 1 << 3,	/* file there */
 	FIO_FILE_EXTEND		= 1 << 4,	/* needs extend */
 	FIO_FILE_NOSORT		= 1 << 5,	/* don't sort verify blocks */
+	FIO_FILE_DONE		= 1 << 6,	/* io completed to this file */
 };
 
 /*