ktls: Fix non-inplace TLS 1.3 encryption.

Copy the iovec for the trailer from the proper place. This is the same
fix for CBC encryption from ff6a7e4ba6bf.

Reported by: gallatin
Reviewed by: gallatin, markj
Fixes: 49f6925ca
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29177