Add support for backwards holes

If you did:

bs=4k
rw=read:-8k

you would essentially rewind by 8k after reading 4k, causing
the read to be sequentially backwards.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.h b/fio.h
index 0c86f28..8401eda 100644
--- a/fio.h
+++ b/fio.h
@@ -254,7 +254,7 @@
 	unsigned int rw_seq;
 	unsigned int kb_base;
 	unsigned int ddir_seq_nr;
-	unsigned long ddir_seq_add;
+	long ddir_seq_add;
 	unsigned int iodepth;
 	unsigned int iodepth_low;
 	unsigned int iodepth_batch;