Add support for KTLS in the Chelsio TOE module.

This adds a TOE hook to allocate a KTLS session. It also recognizes
TLS mbufs in the socket buffer and sends those to the NIC using a TLS
work request to encrypt the record before segmenting it.

TOE TLS support must be enabled via the dev.t6nex.<N>.tls sysctl in
addition to enabling KTLS.

Reviewed by: np, gallatin
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D21891