Fortuna: trivial static variable cleanup

Remove unnecessary use of function-local static variable. 32 bytes is
small enough to live on the stack.

Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16937
(cherry picked from commit 494dda455cd1dad0277a35e292735243f92ec10a)