LinuxKPI: add utsname for init_utsname() with release

A wireless dirver is requesting release from the result of
init_utsname(). Populate the field on startup.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40248

(cherry picked from commit ad513b4dba3e330b6f2714bdde4d864898c4222e)