tun: reserves space for network in skb

The tun driver allocates skb's to hold data from user and then passes
the data into the network stack as received data. Most network devices
allocate the receive skb with routines like dev_alloc_skb() that reserves
additional space for use by network protocol stack but tun does not.

Because of the lack of padding, when the packet is passed through bridge
netfilter a new skb has to be allocated.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -584,7 +584,7 @@ static __inline__ ssize_t tun_get_user(struct tun_struct *tun,
 {
 	struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
 	struct sk_buff *skb;
-	size_t len = count, align = 0;
+	size_t len = count, align = NET_SKB_PAD;
 	struct virtio_net_hdr gso = { 0 };
 	int offset = 0;
 
@@ -614,7 +614,7 @@ static __inline__ ssize_t tun_get_user(struct tun_struct *tun,
 	}
 
 	if ((tun->flags & TUN_TYPE_MASK) == TUN_TAP_DEV) {
-		align = NET_IP_ALIGN;
+		align += NET_IP_ALIGN;
 		if (unlikely(len < ETH_HLEN ||
 			     (gso.hdr_len && gso.hdr_len < ETH_HLEN)))
 			return -EINVAL;