nat64: T6627: call check_kmod within standard config function (#3931)

Functions called from config scripts outside of the standard functions
get_config/verify/generate/apply will not be called when run under
configd. Move as appropriate for the general config script structure and
the specific script requirements.

(cherry picked from commit aeb51976ea23d68d35685bdaa535042a05016185)

Co-authored-by: John Estabrook <jestabro@vyos.io>