Full readonly check

Both in core and in engines. To the extent possible, this should catch
even fio errors.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/libaio.c b/engines/libaio.c
index f8990c1..8f67711 100644
--- a/engines/libaio.c
+++ b/engines/libaio.c
@@ -86,6 +86,8 @@
 {
 	struct libaio_data *ld = td->io_ops->data;
 
+	fio_ro_check(td, io_u);
+
 	if (ld->iocbs_nr == (int) td->o.iodepth)
 		return FIO_Q_BUSY;