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/net.c b/engines/net.c
index 0adc006..552ad0b 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -198,6 +198,8 @@
 	struct netio_data *nd = td->io_ops->data;
 	int ret;
 
+	fio_ro_check(td, io_u);
+
 	if (io_u->ddir == DDIR_WRITE) {
 		if (nd->use_splice)
 			ret = fio_netio_splice_out(td, io_u);