Unprotected modification of ng_iface(4) private data leads to kernel panic.
Fix a race with per-node read-mostly lock and refcounting for a hook.

PR: 220076
Tested by: peixoto.cassiano
Approved by: avg (mentor), mav (mentor)
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D12435