netfilter: nf_queue: fix queueing of bridged gro skbs

When trying to nf_queue GRO/GSO skbs, nf_queue uses skb_gso_segment
to split the skb.

However, if nf_queue is called via bridge netfilter, the mac header
won't be preserved -- packets will thus contain a bogus mac header.

Fix this by setting skb->data to the mac header when skb->nf_bridge
is set and restoring skb->data afterwards for all segments.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>