af_packet: Avoid cache line dirtying

While doing multiple captures, I found af_packet was dirtying cache line
containing its prot_hook.

This slow down machines where several cpus are necessary to handle capture
traffic, as each prot_hook is traversed for each packet coming in or out
the host.

This patches moves "struct packet_type prot_hook" to the end of
packet_sock, and uses a ____cacheline_aligned_in_smp to make sure
this remains shared by all cpus.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -188,7 +188,6 @@ struct packet_sock {
 	struct packet_ring_buffer	tx_ring;
 	int			copy_thresh;
 #endif
-	struct packet_type	prot_hook;
 	spinlock_t		bind_lock;
 	struct mutex		pg_vec_lock;
 	unsigned int		running:1,	/* prot_hook is attached*/
@@ -204,6 +203,7 @@ struct packet_sock {
 	unsigned int		tp_reserve;
 	unsigned int		tp_loss:1;
 #endif
+	struct packet_type	prot_hook ____cacheline_aligned_in_smp;
 };
 
 struct packet_skb_cb {