ipv6: Add IPv6 multipath notification for route delete

If all the nexthops of a multipath route are being deleted, send one
notification for the entire route, instead of one per-nexthop.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>