netlink: Fix up link down behaviour

netlink: Fix up link down behaviour

Partially reverts 04f92904ea6c41517ff7154910c16ef4c3bc646b
 "userv-ipif: Always request routes from userv, regardless of link quality"

It turns out that this check is necessary to avoid bringing up a route
for a "netlink" stanza in the configuration file which is never used.

In particular, this avoids bringing up a netlink for (a) sites which
are not mentioned in the config file (b) the site on which secnet is
running.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>