ip_gre: erspan device should keep dst

The patch 'ip_gre: ipgre_tap device should keep dst' fixed
the issue ipgre_tap dev mtu couldn't be updated in tx path.

The same fix is needed for erspan as well.

Fixes: 84e54fe0 ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1254,6 +1254,7 @@ static int erspan_tunnel_init(struct net_device *dev)
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
 	dev->priv_flags		|= IFF_LIVE_ADDR_CHANGE;
+	netif_keep_dst(dev);
 
 	return ip_tunnel_init(dev);
 }