Fix bug in sha256 verify

It was using twice as large data block as it should have been,
causing verify failures.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/verify.h b/verify.h
index c12bc7d..be98c54 100644
--- a/verify.h
+++ b/verify.h
@@ -37,7 +37,7 @@
 	uint8_t sha512[128];
 };
 struct vhdr_sha256 {
-	uint8_t sha256[128];
+	uint8_t sha256[64];
 };
 struct vhdr_sha1 {
 	uint32_t sha1[5];