dn_route: set rt neigh to blackhole_netdev instead of loopback_dev in ifdown

Like other places in ipv4/6 dst ifdown, change to use blackhole_netdev
instead of pernet loopback_dev in dn dst ifdown.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/0cdf10e5a4af509024f08644919121fb71645bc2.1652751029.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>