opencrypto: Embed the driver softc in the session structure

Store the driver softc below the fields owned by opencrypto. This is
a bit simpler and saves a pointer dereference when fetching the driver
softc when processing a request.

Get rid of the crypto session UMA zone. Session allocations are
frequent or performance-critical enough to warrant a dedicated zone.

No functional change intended.

Reviewed by: cem, jhb
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D28158