[NETFILTER]: bridge-netfilter: fix net_device refcnt leaks

When packets are flood-forwarded to multiple output devices, the
bridge-netfilter code reuses skb->nf_bridge for each clone to store
the bridge port. When queueing packets using NFQUEUE netfilter takes
a reference to skb->nf_bridge->physoutdev, which is overwritten
when the packet is forwarded to the second port. This causes
refcount unterflows for the first device and refcount leaks for all
others. Additionally this provides incorrect data to the iptables
physdev match.

Unshare skb->nf_bridge by copying it if it is shared before assigning
the physoutdev device.

Reported, tested and based on initial patch by
Jan Christoph Nordholz <hesso@pool.math.tu-berlin.de>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>