MFC r343418:

pf: Fix use-after-free of counters

When cleaning up a vnet we free the counters in V_pf_default_rule and
V_pf_status from shutdown_pf(), but we can still use them later, for example
through pf_purge_expired_src_nodes().

Free them as the very last operation, as they rely on nothing else themselves.

PR: 235097