KTLS: Move OCF function pointers out of ktls_session.

Instead, create a switch structure private to ktls_ocf.c and store a
pointer to the switch in the ocf_session. This will permit adding an
additional function pointer needed for NIC TLS RX without further
bloating ktls_session.

Reviewed by: hselasky
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35011