netfilter: bridge: add nf_afinfo to enable queuing to userspace

This just adds and registers a nf_afinfo for the ethernet
bridge, which enables queuing to userspace for the AF_BRIDGE
family. No checksum computation is done.

Signed-off-by: Stephane Bryant <stephane.ml.bryant@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>