netfilter: bridge: start splitting mask into public/private chunks

->mask is a bit info field that mixes various use cases.

In particular, we have flags that are mutually exlusive, and flags that
are only used within br_netfilter while others need to be exposed to
other parts of the kernel.

Remove BRNF_8021Q/PPPoE flags.  They're mutually exclusive and only
needed within br_netfilter context.

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