net: ip_gre: always reports o_key to userspace

Erspan protocol (version 1 and 2) relies on o_key to configure
session id header field. However TUNNEL_KEY bit is cleared in
erspan_xmit since ERSPAN protocol does not set the key field
of the external GRE header and so the configured o_key is not reported
to userspace. The issue can be triggered with the following reproducer:

$ip link add erspan1 type erspan local 192.168.0.1 remote 192.168.0.2 \
    key 1 seq erspan_ver 1
$ip link set erspan1 up
$ip -d link sh erspan1

erspan1@NONE: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc pfifo_fast state UNKNOWN mode DEFAULT
  link/ether 52:aa:99:95:9a:b5 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500
  erspan remote 192.168.0.2 local 192.168.0.1 ttl inherit ikey 0.0.0.1 iseq oseq erspan_index 0

Fix the issue adding TUNNEL_KEY bit to the o_flags parameter in
ipgre_fill_info

Fixes: 84e54fe0 ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1455,12 +1455,17 @@ static int ipgre_fill_info(struct sk_buff *skb, const struct net_device *dev)
 {
 	struct ip_tunnel *t = netdev_priv(dev);
 	struct ip_tunnel_parm *p = &t->parms;
+	__be16 o_flags = p->o_flags;
+
+	if ((t->erspan_ver == 1 || t->erspan_ver == 2) &&
+	    !t->collect_md)
+		o_flags |= TUNNEL_KEY;
 
 	if (nla_put_u32(skb, IFLA_GRE_LINK, p->link) ||
 	    nla_put_be16(skb, IFLA_GRE_IFLAGS,
 			 gre_tnl_flags_to_gre_flags(p->i_flags)) ||
 	    nla_put_be16(skb, IFLA_GRE_OFLAGS,
-			 gre_tnl_flags_to_gre_flags(p->o_flags)) ||
+			 gre_tnl_flags_to_gre_flags(o_flags)) ||
 	    nla_put_be32(skb, IFLA_GRE_IKEY, p->i_key) ||
 	    nla_put_be32(skb, IFLA_GRE_OKEY, p->o_key) ||
 	    nla_put_in_addr(skb, IFLA_GRE_LOCAL, p->iph.saddr) ||