[DECNET]: Fix for routing bug

This patch fixes a bug in the DECnet routing code where we were
selecting a loopback device in preference to an outward facing device
even when the destination was known non-local. This patch should fix
the problem.

Signed-off-by: Patrick Caulfield <patrick@tykepenguin.com>
Signed-off-by: Steven Whitehouse <steve@chygwyn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>