netlink: Fix C++ compile errors

Allow these files to be included in C++ programs with careful casting to
the proper type, like C++ wants (and in a way that also works for C).

MFC After: 1 week
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1245

(cherry picked from commit ff92493a4f6504c49a6c84ec65053f493ff5d708)