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/io_u.c b/io_u.c
index f2406e8..0d55c09 100644
--- a/io_u.c
+++ b/io_u.c
@@ -681,10 +681,15 @@
 	}
 
 	f = td->file_service_file;
-	if (f && (f->flags & FIO_FILE_OPEN) && td->file_service_left--)
-		goto out;
+	if (f && (f->flags & FIO_FILE_OPEN)) {
+		if (td->o.file_service_type == FIO_FSERVICE_SEQ)
+			goto out;
+		if (td->file_service_left--)
+			goto out;
+	}
 
-	if (td->o.file_service_type == FIO_FSERVICE_RR)
+	if (td->o.file_service_type == FIO_FSERVICE_RR ||
+	    td->o.file_service_type == FIO_FSERVICE_SEQ)
 		f = get_next_file_rr(td, FIO_FILE_OPEN, FIO_FILE_CLOSING);
 	else
 		f = get_next_file_rand(td, FIO_FILE_OPEN, FIO_FILE_CLOSING);