geliboot: Use the correct IV length for AES-XTS.

• Use AES_XTS_IV_LEN instead of the key length as the IV length.
• Use G_ELI_IVKEYLEN as the size of the zeroed iv[] array in g_eli_crypto_cipher() to match geli_io().

PR: 261172
Reported by: Malcolm Matalka <mmatalka@gmail.com>, mikael
Reviewed by: markj
Sponsored by: FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33884