ktls: Change struct ktls_session.cipher to an OCF-specific type.

As a followup to SW KTLS assuming an OCF backend, rename
struct ocf_session to struct ktls_ocf_session and forward
declare it in <sys/ktls.h> to use as the type of
struct ktls_session.cipher.

Reviewed by: gallatin, hselasky
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32565