packet: add extended BPF fanout mode

Add fanout mode PACKET_FANOUT_EBPF that accepts an en extended BPF
program to select a socket.

Update the internal eBPF program by passing to socket option
SOL_PACKET/PACKET_FANOUT_DATA a file descriptor returned by bpf().

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_packet.h

@@ -64,6 +64,7 @@ struct sockaddr_ll {
 #define PACKET_FANOUT_RND		4
 #define PACKET_FANOUT_QM		5
 #define PACKET_FANOUT_CBPF		6
+#define PACKET_FANOUT_EBPF		7
 #define PACKET_FANOUT_FLAG_ROLLOVER	0x1000
 #define PACKET_FANOUT_FLAG_DEFRAG	0x8000