tun: fix inability to set offloads after disabling them via ethtool

Added missing logic in tun driver, which prevents apps to set
offloads using tun ioctl, if offloads were previously disabled via ethtool

Signed-off-by: Yaroslav Isakov <yaroslav.isakov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 34cc3c5..cc88cd7 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1931,6 +1931,8 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
 		return -EINVAL;
 
 	tun->set_features = features;
+	tun->dev->wanted_features &= ~TUN_USER_FEATURES;
+	tun->dev->wanted_features |= features;
 	netdev_update_features(tun->dev);
 
 	return 0;