cryptosoft: Stop single-threading requests within a session.

All of the request handlers no longer modify session state, so remove
the mutex limiting operations to one per session. In addition, change
the pointer to the session state passed to process callbacks to const.

Suggested by: mjg
Reviewed by: mjg, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33317