Add fill_device option

Add fill_device option. Causes fio to write until ENOSPC occurs (assuming
rw=write).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.c b/fio.c
index e9b27d9..1c79d68 100644
--- a/fio.c
+++ b/fio.c
@@ -568,6 +568,10 @@
 		}
 	}
 
+	if (td->o.fill_device && td->error == ENOSPC) {
+		td->error = 0;
+		td->terminate = 1;
+	}
 	if (!td->error) {
 		struct fio_file *f;