KTLS: Construct IV directly in crp.crp_iv for TLS 1.3 AEAD encryption.

Previously this used a temporary nonce[] buffer. The decrypt hook for
TLS 1.3 as well as the hooks for TLS 1.2 already constructed the IV
directly in crp.crp_iv.

Reviewed by: hselasky
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35027