kTLS support for TLS 1.3

TLS 1.3 requires a few changes because 1.3 pretends to be 1.2
with a record type of application data. The "real" record type is
then included at the end of the user-supplied plaintext
data. This required adding a field to the mbuf_ext_pgs struct to
save the record type, and passing the real record type to the
sw_encrypt() ktls backend functions.

Reviewed by: jhb, hselasky
Sponsored by: Netflix
Differential Revision: D21801