net: TX_RING and packet mmap

New packet socket feature that makes packet socket more efficient for
transmission.

- It reduces number of system call through a PACKET_TX_RING mechanism,
  based on PACKET_RX_RING (Circular buffer allocated in kernel space
  which is mmapped from user space).

- It minimizes CPU copy using fragmented SKB (almost zero copy).

Signed-off-by: Johann Baudy <johann.baudy@gnu-log.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -203,6 +203,9 @@ struct skb_shared_info {
 #ifdef CONFIG_HAS_DMA
 	dma_addr_t	dma_maps[MAX_SKB_FRAGS + 1];
 #endif
+	/* Intermediate layers must ensure that destructor_arg
+	 * remains valid until skb destructor */
+	void *		destructor_arg;
 };
 
 /* We divide dataref into two halves.  The higher 16 bits hold references