safexcel: Maintain per-session context records

The context record contains key material precomputed by the driver at
session creation time. Rather than storing various components of the
context record in each session, go a bit further and store the full
context record image so that safexcel_process() can simply copy the
image into each request submitted to the hardware. This simplifies the
data path and eliminates a bunch of unnecessary conditional logic that
was getting executed for each request.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC (Netgate)