fscrypt: zeroize fscrypt_info before freeing

memset the struct fscrypt_info to zero before freeing.  This isn't
really needed currently, since there's no secret key directly in the
fscrypt_info.  But there's a decent chance that someone will add such a
field in the future, e.g. in order to use an API that takes a raw key
such as siphash().  So it's good to do this as a hardening measure.

Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keysetup.c

@@ -327,6 +327,7 @@ static void put_crypt_info(struct fscrypt_info *ci)
 			key_invalidate(key);
 		key_put(key);
 	}
+	memzero_explicit(ci, sizeof(*ci));
 	kmem_cache_free(fscrypt_info_cachep, ci);
 }