Only attempt zone switch if we have zones enabled

Just an offset isn't enough.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/io_u.c b/io_u.c
index d794f01..161c2cb 100644
--- a/io_u.c
+++ b/io_u.c
@@ -668,7 +668,7 @@
 	/*
 	 * See if it's time to switch to a new zone
 	 */
-	if (td->zone_bytes >= td->o.zone_size) {
+	if (td->zone_bytes >= td->o.zone_size && td->o.zone_skip) {
 		td->zone_bytes = 0;
 		io_u->file->file_offset += td->o.zone_range + td->o.zone_skip;
 		io_u->file->last_pos = io_u->file->file_offset;