After investigation it turns out there were two issues.
1) Phonet was not implementing network devices but was using register_pernet_device
instead of register_pernet_subsys.
This was allowing there to be cases when phonenet was not initialized and
the phonet net_generic was not set for a network namespace when network
device events were being reported on the netdevice_notifier for a network
namespace leading to the oops above.
2) phonet_exit_net was implementing a confusing and special case of handling all
network devices from going away that it was hard to see was correct, and would
only occur when the phonet module was removed.
Now that unregister_netdevice_notifier has been modified to synthesize unregistration
events for the network devices that are extant when called this confusing special
case in phonet_exit_net is no longer needed.