kern_sysctl: Fix printing function name in the re-use sysctl leaf warning

The helper function sysctl_warn_reuse() is intended to print the name of
the caller rather than that of itself.

PR: 221853
Fixes: 4ae2ade11426 Enhance debugibility of sysctl leaf re-use warnings
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48645