netfilter: bridge: free nf_bridge info on xmit

nf_bridge information is only needed for -m physdev, so we can always free
it after POST_ROUTING.  This has the advantage that allocation and free will
typically happen on the same cpu.

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