fortuna: Drop global lock to zero stack variables

Also drop explicit zeroing of hash context -- hash finish() operation is
expected to do this.

PR: 230877
Suggested by: delphij@
Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16986
(cherry picked from commit 7be4093a844e80a591221e93d03f3daf6a972be3)