md5: fix bug in hash size

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/verify.h b/verify.h
index be98c54..a4a8cfe 100644
--- a/verify.h
+++ b/verify.h
@@ -31,7 +31,7 @@
 };
 
 struct vhdr_md5 {
-	uint32_t md5_digest[16];
+	uint32_t md5_digest[4];
 };
 struct vhdr_sha512 {
 	uint8_t sha512[128];