kern_sysctl: Fix sysctl leaf re-use warnings

[zlei]

Summary

The original PR 221853 actually requested to distinguish which function (the caller) emits the re-use warning, while the implementation sysctl_warn_reuse() has the parameter func (function name ) passed in but not used. That renders the warning message less useful.

Fix: 4ae2ade11426 Enhance debugibility of sysctl leaf re-use warnings
MFC after: 1 week

Test Plan

Before this change:

# sysctl sysctl.reuse_test
sysctl_warn_reuse: can't re-use a leaf (sysctl.reuse_test)!

After the change:

# sysctl sysctl.reuse_test
sysctl_reuse_test: can't re-use a leaf (sysctl.reuse_test)!