Store the AAD in a separate buffer for KTLS.

For TLS 1.2 this permits reusing one of the existing iovecs without
always having to duplicate both.

While here, only duplicate the output iovec for TLS 1.3 if it will be
used.

Reviewed by: gallatin
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D25291