ipfilter: Add missing kenv fetch

When a module the environment must be explicitly fetched.

Fixes: d9788eabffa4
PR: 291548
Noted by: markj
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D54242

(cherry picked from commit a6ea80bc917510b5e056cc5a29b62dfd7b39d068)