netfront: Unregister inetdev notifiers on failure

netfront: Unregister inetdev notifiers on failure
If you attempt to modprobe the pv-on-hvm netfront driver on a machine
not running under Xen (say, bare-metal, or under another hypervisor), the
netfront code correctly returns an ENODEV and fails to load.  However, if you
then shutdown that machine, you will oops while tearing down the network.
This is because we forget to unregister the the inetaddr_notifier on failure,
and so the kernel takes a fatal page fault.  The attached patch just unregisters
the notifier on failure, and solves the problem for me.

Signed-off-by: Chris Lalancette <clalance@redhat.com>