Add some support for a verify backlog

Instead of writing everything and then verifying everything,
allow a job to specify incremental verify stages. This can
help reduce memory consumption of fio, since we don't have
to store a huge number of verify entries to be processed
when the write phase is complete.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/verify.c b/verify.c
index c894b60..6932cfc 100644
--- a/verify.c
+++ b/verify.c
@@ -748,8 +748,10 @@
 
 		ipo = rb_entry(n, struct io_piece, rb_node);
 		rb_erase(n, &td->io_hist_tree);
+		td->io_hist_len--;
 	} else if (!flist_empty(&td->io_hist_list)) {
 		ipo = flist_entry(td->io_hist_list.next, struct io_piece, list);
+		td->io_hist_len--;
 		flist_del(&ipo->list);
 	}