net: route: Verify if neighbor entry is in use when iterating

Verify the `ref` value of the neighbor entry in `net_route_foreach()`,
so that it only executes the callback on routes in use. Otherwise, the
function could call the callback for the route that has already been
deleted.

This could be encountered when executing `net route` shell command,
which printed the already deleted routes along the existing ones.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>