Fix RSAZ's OPENSSL_cleanse.

https://boringssl-review.googlesource.com/28584 switched RSAZ's buffer
to being externally-allocated, which means the OPENSSL_cleanse needs to
be tweaked to match.

Change-Id: I0a7307ac86aa10933d10d380ef652c355fed3ee9
Reviewed-on: https://boringssl-review.googlesource.com/c/34191
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/fipsmodule/bn/rsaz_exp.c

@@ -233,7 +233,7 @@ void RSAZ_1024_mod_exp_avx2(BN_ULONG result_norm[16],
 
   rsaz_1024_red2norm_avx2(result_norm, result);
 
-  OPENSSL_cleanse(storage, sizeof(storage));
+  OPENSSL_cleanse(storage, MOD_EXP_CTIME_STORAGE_LEN * sizeof(BN_ULONG));
 }
 
 #endif  // OPENSSL_X86_64