tun/kerndat: Keep going with CONFIG_TUN=n

check_tun_netns_cr() should return 0 on CONFIG_TUN=n kernels.
That is, when we fail to open tun device.

Fixes: #496
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/tun.c

@@ -74,17 +74,18 @@ int check_tun_cr(int no_tun_err)
 
 int check_tun_netns_cr(bool *result)
 {
-	bool val;
+	bool val = false;
 	int tun;
 
 	tun = open(TUN_DEV_GEN_PATH, O_RDONLY);
 	if (tun < 0) {
 		pr_perror("Unable to create tun");
-		return -1;
+		goto out;
 	}
 	check_has_netns_ioc(tun, &val, "tun");
 	close(tun);
 
+out:
 	if (result)
 		*result = val;