MFC r323873, r324081: 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: mav (mentor)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D12435