decnet: Do not build routes to devices without decnet private data.

[ Upstream commit a36a0d4008488fa545c74445d69eaf56377d5d4e ]

In particular, make sure we check for decnet private presence
for loopback devices.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>