net: rtnetlink: ignore NETDEV_PRE_TYPE_CHANGE in rtnetlink_event()

Ignore the new NETDEV_PRE_TYPE_CHANGE event in rtnetlink_event() since
there have been no changes userspace needs to be notified of.

Also add a comment to the netdev notifier event definitions to remind
people to update the exclusion list when adding new event types.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>