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/init.c b/init.c
index b4a0cbb..62c7dc2 100644
--- a/init.c
+++ b/init.c
@@ -380,6 +380,7 @@
 	profile_add_hooks(td);
 
 	td->thread_number = thread_number;
+	td->subjob_number = 0;
 
 	if (jobname)
 		td->o.name = strdup(jobname);
@@ -1288,6 +1289,7 @@
 		td_new->o.numjobs = 1;
 		td_new->o.stonewall = 0;
 		td_new->o.new_group = 0;
+		td_new->subjob_number = numjobs;
 
 		if (file_alloced) {
 			if (td_new->files) {