packet: Add fanout support.

Fanouts allow packet capturing to be demuxed to a set of AF_PACKET
sockets.  Two fanout policies are implemented:

1) Hashing based upon skb->rxhash

2) Pure round-robin

An AF_PACKET socket must be fully bound before it tries to add itself
to a fanout.  All AF_PACKET sockets trying to join the same fanout
must all have the same bind settings.

Fanouts are identified (within a network namespace) by a 16-bit ID.
The first socket to try to add itself to a fanout with a particular
ID, creates that fanout.  When the last socket leaves the fanout
(which happens only when the socket is closed), that fanout is
destroyed.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_packet.h

@@ -49,6 +49,10 @@ struct sockaddr_ll {
 #define PACKET_VNET_HDR			15
 #define PACKET_TX_TIMESTAMP		16
 #define PACKET_TIMESTAMP		17
+#define PACKET_FANOUT			18
+
+#define PACKET_FANOUT_HASH		0
+#define PACKET_FANOUT_LB		1
 
 struct tpacket_stats {
 	unsigned int	tp_packets;