Add 'sequential' file_service_type

This service type will keep a file open until IO to it is completely
done, before moving on to the next available file.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index e74836f..11338eb 100644
--- a/fio.h
+++ b/fio.h
@@ -657,11 +657,13 @@
 };
 
 /*
- * roundrobin available files, or choose one at random.
+ * roundrobin available files, or choose one at random, or do each one
+ * serially.
  */
 enum {
 	FIO_FSERVICE_RANDOM	= 1,
 	FIO_FSERVICE_RR		= 2,
+	FIO_FSERVICE_SEQ	= 3,
 };
 
 /*