netdev-linux: Fix pairing of rtnetlink register and unregister calls.

netdev-linux: Fix pairing of rtnetlink register and unregister calls.

netdev_linux_create() called rtnetlink_notifier_register() for both system
and internal devices, but netdev_linux_destroy() only did the reverse
accounting for system devices.  This fixes the pairing.

This isn't really much of a bug, since it would only cause the notifier to
be active unnecessarily (not to be removed even though it was needed).  At
most it was a missed opportunity for optimization, but I don't think that
optimization would ever happen anyway.

Found with valgrind --leak-check=full --show-reachable=yes.