cryptosoft: Allocate cipher contexts on the stack during operations.

As is done with authentication contexts, allocate cipher contexts on
the stack while completing requests. This permits safely dispatching
concurrent requests on a single session. The cipher context in the
session is now only allocated when a session key is provided during
session setup to serve as a template to initialize the on-stack
context similar to auth operations.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33198