bridge: Use BR_INPUT_SKB_CB on xmit path

this patch makes BR_INPUT_SKB_CB available on the xmit path so
that we could avoid passing the br pointer around for the purpose
of collecting device statistics.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>