Fortuna: Clean up reseeding key material to closer match FS&K

When reseeding, only incorporate actual key material. Do not include e.g.
the derived key schedules or other AES context.

I don't think the extra material was harmful here, just not beneficial.

Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16934