In your ensureAuthenticated method save the return url in the session like this:

...
req.session.returnTo = req.originalUrl; 
res.redirect('/login');
...

Then you can update your passport.authenticate route to something like:

app.get('/auth/google/return', passport.authenticate('google'), function(req, res) {
    res.redirect(req.session.returnTo || '/');
    delete req.session.returnTo;
});