bridge: stp: ensure mac header is set

commit bf9ae538
(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>