geneve: fix an uninitialized value in geneve_changelink()

geneve_nl2info() sets 'df' conditionally, so we have to
initialize it by copying the value from existing geneve
device in geneve_changelink().

Fixes: 56c09de3 ("geneve: allow changing DF behavior after creation")
Reported-by: syzbot+7ebc2e088af5e4c0c9fa@syzkaller.appspotmail.com
Cc: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/geneve.c

@@ -1615,11 +1615,11 @@ static int geneve_changelink(struct net_device *dev, struct nlattr *tb[],
 			     struct netlink_ext_ack *extack)
 {
 	struct geneve_dev *geneve = netdev_priv(dev);
+	enum ifla_geneve_df df = geneve->df;
 	struct geneve_sock *gs4, *gs6;
 	struct ip_tunnel_info info;
 	bool metadata;
 	bool use_udp6_rx_checksums;
-	enum ifla_geneve_df df;
 	bool ttl_inherit;
 	int err;