[NETFILTER]: bridge: fix double POSTROUTING hook invocation

Packets routed between bridges have the POST_ROUTING hook invoked
twice since bridging mistakes them for bridged packets because
they have skb->nf_bridge set.

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