vxlan: Notify drivers for listening UDP port changes

This patch adds two more ndo ops: ndo_add_rx_vxlan_port() and
ndo_del_rx_vxlan_port().

Drivers can get notifications through the above functions about changes
of the UDP listening port of VXLAN. Also, when physical ports come up,
now they can call vxlan_get_rx_port() in order to obtain the port number(s)
of the existing VXLAN interface in case they already up before them.

This information about the listening UDP port would be used for VXLAN
related offloads.

A big thank you to John Fastabend (john.r.fastabend@intel.com) for his
input and his suggestions on this patch set.

CC: John Fastabend <john.r.fastabend@intel.com>
CC: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Joseph Gasparakis <joseph.gasparakis@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/vxlan.h b/include/net/vxlan.h
index e09c40b..2d64d3c 100644
--- a/include/net/vxlan.h
+++ b/include/net/vxlan.h
@@ -36,4 +36,5 @@
 
 __be16 vxlan_src_port(__u16 port_min, __u16 port_max, struct sk_buff *skb);
 
+void vxlan_get_rx_port(struct net_device *netdev);
 #endif