net: mpls: Fix notifications when deleting a device

There are various problems related to netlink notifications for mpls route
changes in response to interfaces being deleted:
* delete interface of only nexthop
	DELROUTE notification is missing RTA_OIF attribute
* delete interface of non-last nexthop
	NEWROUTE notification is missing entirely
* delete interface of last nexthop
	DELROUTE notification is missing nexthop

All of these problems stem from the fact that existing routes are modified
in-place before sending a notification. Restructure mpls_ifdown() to avoid
changing the route in the DELROUTE cases and to create a copy in the
NEWROUTE case.

Fixes: f8efb73c

 ("mpls: multipath route support")
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>