net: reset mac header in dev_start_xmit()

On 64 bit arches :

There is a off-by-one error in qdisc_pkt_len_init() because
mac_header is not set in xmit path.

skb_mac_header() returns an out of bound value that was
harmless because hdr_len is an 'unsigned int'

On 32bit arches, the error is abysmal.

This patch is also a prereq for "macvlan: add multicast filter"

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Ben Greear <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/core/dev.c

@@ -2835,6 +2835,8 @@ int dev_queue_xmit(struct sk_buff *skb)
 	struct Qdisc *q;
 	int rc = -ENOMEM;
 
+	skb_reset_mac_header(skb);
+
 	/* Disable soft irqs for various locks below. Also
 	 * stops preemption for RCU.
 	 */