When tearing down a VNET, netgraph sends shutdown messages to all of the
nodes before detaching interfaces (SI_SUB_NETGRAPH comes before
SI_SUB_INIT_IF in teardown order). ng_ether nodes handle this by
destroying themselves without detaching from the parent ifnet. Then,
when ifnets go away they detach their ng_ether nodes again, triggering a
Handle this by modifying ng_ether_shutdown() to detach from the ifnet.
If the shutdown was triggered by an ifnet being destroyed, we will clear
priv->ifp in the ng_ether detach callback, so priv->ifp may be NULL.
Also get rid of the printf in vnet_netgraph_uninit(). It can be
triggered trivially by ng_ether since ng_ether_shutdown() persists the
node unless NG_REALLY_DIE is set.
Reported by: KASAN