net: ip6_gre: Split up ip6gre_newlink()

Extract from ip6gre_newlink() a reusable function
ip6gre_newlink_common(). The ip6gre_tnl_link_config() call needs to be
made customizable for ERSPAN, thus reorder it with calls to
ip6_tnl_change_mtu() and dev_hold(), and extract the whole tail to the
caller, ip6gre_newlink(). Thus enable an ERSPAN-specific _newlink()
function without a lot of duplicity.

Fixes: 5a963eb6 ("ip6_gre: Add ERSPAN native tunnel support")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1861,7 +1861,7 @@ static bool ip6gre_netlink_encap_parms(struct nlattr *data[],
 	return ret;
 }
 
-static int ip6gre_newlink(struct net *src_net, struct net_device *dev,
+static int ip6gre_newlink_common(struct net *src_net, struct net_device *dev,
 				 struct nlattr *tb[], struct nlattr *data[],
 				 struct netlink_ext_ack *extack)
 {
@@ -1900,18 +1900,30 @@ static int ip6gre_newlink(struct net *src_net, struct net_device *dev,
 	if (err)
 		goto out;
 
-	ip6gre_tnl_link_config(nt, !tb[IFLA_MTU]);
-
 	if (tb[IFLA_MTU])
 		ip6_tnl_change_mtu(dev, nla_get_u32(tb[IFLA_MTU]));
 
 	dev_hold(dev);
-	ip6gre_tunnel_link(ign, nt);
 
 out:
 	return err;
 }
 
+static int ip6gre_newlink(struct net *src_net, struct net_device *dev,
+			  struct nlattr *tb[], struct nlattr *data[],
+			  struct netlink_ext_ack *extack)
+{
+	int err = ip6gre_newlink_common(src_net, dev, tb, data, extack);
+	struct ip6_tnl *nt = netdev_priv(dev);
+	struct net *net = dev_net(dev);
+
+	if (!err) {
+		ip6gre_tnl_link_config(nt, !tb[IFLA_MTU]);
+		ip6gre_tunnel_link(net_generic(net, ip6gre_net_id), nt);
+	}
+	return err;
+}
+
 static int ip6gre_changelink(struct net_device *dev, struct nlattr *tb[],
 			     struct nlattr *data[],
 			     struct netlink_ext_ack *extack)