Add core for verify_dump option

The verify_dump option is defined and documented, but does nothing.
This one-liner enables the option.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/verify.c b/verify.c
index c450e88..43dd392 100644
--- a/verify.c
+++ b/verify.c
@@ -269,6 +269,9 @@
 	struct io_u dummy;
 	void *buf;
 
+	if (!td->o.verify_dump)
+		return;
+
 	/*
 	 * Dump the contents we just read off disk
 	 */