Fix random location generation with offset != 0

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 8f10bfd..a56a6f2 100644
--- a/io_u.c
+++ b/io_u.c
@@ -44,7 +44,7 @@
 	unsigned int blocks;
 	unsigned int nr_blocks;
 
-	block = io_u->offset / (unsigned long long) min_bs;
+	block = (io_u->offset - f->file_offset) / (unsigned long long) min_bs;
 	blocks = 0;
 	nr_blocks = (io_u->buflen + min_bs - 1) / min_bs;