netfilter: bridge: convert br_netfilter to NF_DROP_REASON

errno is 0 because these hooks are called from prerouting and forward.
There is no socket that the errno would ever be propagated to.

Other netfilter modules (e.g. nf_nat, conntrack, ...) can be converted
in a similar way.

Signed-off-by: Florian Westphal <fw@strlen.de>