Allow reset of offset_increment counter

Reset offset_increment only within a sub-group of jobs, eg when
numjobs=x is used to automatically group jobs together.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.h b/fio.h
index c694f2c..dfbad6d 100644
--- a/fio.h
+++ b/fio.h
@@ -102,6 +102,7 @@
 	char verror[FIO_VERROR_SIZE];
 	pthread_t thread;
 	unsigned int thread_number;
+	unsigned int subjob_number;
 	unsigned int groupid;
 	struct thread_stat ts;