geneve: initialize needed_headroom

Currently the needed_headroom field for the geneve device is left
to the default value.

This patch set it to space required for basic geneve encapsulation,
so that we can avoid the skb head re-allocation on xmit.

This give a 6% speedup for unsegment traffic on geneve tunnel.

v1 -> v2:
  - add ETH_HLEN for the lower device to the needed headroom
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/geneve.c

@@ -1155,7 +1155,7 @@ static int geneve_configure(struct net *net, struct net_device *dev,
 	struct geneve_net *gn = net_generic(net, geneve_net_id);
 	struct geneve_dev *t, *geneve = netdev_priv(dev);
 	bool tun_collect_md, tun_on_same_port;
-	int err;
+	int err, encap_len;
 
 	if (!remote)
 		return -EINVAL;
@@ -1187,6 +1187,14 @@ static int geneve_configure(struct net *net, struct net_device *dev,
 	if (t)
 		return -EBUSY;
 
+	/* make enough headroom for basic scenario */
+	encap_len = GENEVE_BASE_HLEN + ETH_HLEN;
+	if (remote->sa.sa_family == AF_INET)
+		encap_len += sizeof(struct iphdr);
+	else
+		encap_len += sizeof(struct ipv6hdr);
+	dev->needed_headroom = encap_len + ETH_HLEN;
+
 	if (metadata) {
 		if (tun_on_same_port)
 			return -EPERM;