Add nr parameter to file_service_type

Right now we switch for every IO, add a postfix that allows to switch
for every 'x' number of ios.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index a60be10..9d190b1 100644
--- a/fio.h
+++ b/fio.h
@@ -473,6 +473,13 @@
 	 */
 	struct timeval timeout_end;
 	struct itimerval timer;
+
+	/*
+	 * for fileservice, how often to switch to a new file
+	 */
+	unsigned int file_service_nr;
+	unsigned int file_service_left;
+	struct fio_file *file_service_file;
 };
 
 /*