OCF: Add crypto_clonereq().

This function clones an existing crypto request, but associates the
new request with a specified session. The intended use case is for
drivers to be able to fall back to software by cloning a request and
dispatch it to an internally allocated software session.

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33607

(cherry picked from commit 74d3f1b63dbea05038e966cf4bb69a01b0589500)