Inc block in mark_random_map()

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index f2fa859..c33dddf 100644
--- a/io_u.c
+++ b/io_u.c
@@ -78,6 +78,7 @@
 		f->file_map[idx] |= mask;
 		nr_blocks -= this_blocks;
 		blocks += this_blocks;
+		block += this_blocks;
 	}
 
 	if ((blocks * min_bs) < io_u->buflen)