bridge: add new BR_NEIGH_SUPPRESS port flag to suppress arp and nd flood

This patch adds a new bridge port flag BR_NEIGH_SUPPRESS to
suppress arp and nd flood on bridge ports. It implements
rfc7432, section 10.
https://tools.ietf.org/html/rfc7432#section-10
for ethernet VPN deployments. It is similar to the existing
BR_PROXYARP* flags but has a few semantic differences to conform
to EVPN standard. Unlike the existing flags, this new flag suppresses
flood of all neigh discovery packets (arp and nd) to tunnel ports.
Supports both vlan filtering and non-vlan filtering bridges.

In case of EVPN, it is mainly used to avoid flooding
of arp and nd packets to tunnel ports like vxlan.

This patch adds netlink and sysfs support to set this bridge port
flag.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/if_bridge.h b/include/linux/if_bridge.h
index 3cd18ac..316ee11 100644
--- a/include/linux/if_bridge.h
+++ b/include/linux/if_bridge.h
@@ -49,6 +49,7 @@ struct br_ip_list {
 #define BR_MULTICAST_TO_UNICAST	BIT(12)
 #define BR_VLAN_TUNNEL		BIT(13)
 #define BR_BCAST_FLOOD		BIT(14)
+#define BR_NEIGH_SUPPRESS	BIT(15)
 
 #define BR_DEFAULT_AGEING_TIME	(300 * HZ)