bridge: stp: ensure mac header is set

commit bf9ae5386b
(llc: use dev_hard_header) removed the
skb_reset_mac_header call from llc_mac_hdr_init.

This seems fine itself, but br_send_bpdu() invokes ebtables LOCAL_OUT.

We oops in ebt_basic_match() because it assumes eth_hdr(skb) returns
a meaningful result.

Cc: acme@ghostprotocols.net
References: https://bugzilla.kernel.org/show_bug.cgi?id=24532
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_stp_bpdu.c

@@ -50,6 +50,8 @@ static void br_send_bpdu(struct net_bridge_port *p,
 
 	llc_mac_hdr_init(skb, p->dev->dev_addr, p->br->group_addr);
 
+	skb_reset_mac_header(skb);
+
 	NF_HOOK(NFPROTO_BRIDGE, NF_BR_LOCAL_OUT, skb, NULL, skb->dev,
 		dev_queue_xmit);
 }