Use G_ELI_IVKEYLEN as the size of IV in the user test code.

IVs are not the size of keys as a general case.  Most often they are
the size of a single block.

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D33885

sys/geom/eli/g_eli_crypto.c

@@ -112,7 +112,7 @@ g_eli_crypto_cipher(u_int algo, int enc, u_char *data, size_t datasize,
 {
 	EVP_CIPHER_CTX *ctx;
 	const EVP_CIPHER *type;
-	u_char iv[keysize];
+	u_char iv[G_ELI_IVKEYLEN];
 	int outsize;
 
 	assert(algo != CRYPTO_AES_XTS);