net: decnet: fix netdev refcount leaking on error path

On building the route there is an assumption that the destination
could be local. In this case loopback_dev is used to get the address.
If the address is still cannot be retrieved dn_route_output_slow
returns EADDRNOTAVAIL with loopback_dev reference taken.

Cannot find hash for the fixes tag because this code was introduced
long time ago. I don't think that this bug has ever fired but the
patch is done just to have a consistent code base.

Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Link: https://lore.kernel.org/r/1611619334-20955-1-git-send-email-vfedorenko@novek.ru


Signed-off-by: Jakub Kicinski <kuba@kernel.org>