bridge: netfilter: unroll NF_HOOK helper in bridge input path

Replace NF_HOOK() based invocation of the netfilter hooks with a private
copy of nf_hook_slow().

This copy has one difference: it can return the rx handler value expected
by the stack, i.e. RX_HANDLER_CONSUMED or RX_HANDLER_PASS.

This is needed by the next patch to invoke the ebtables
"broute" table via the standard netfilter hooks rather than the custom
"br_should_route_hook" indirection that is used now.

When the skb is to be "brouted", we must return RX_HANDLER_PASS from the
bridge rx input handler, but there is no way to indicate this via
NF_HOOK(), unless perhaps by some hack such as exposing bridge_cb in the
netfilter core or a percpu flag.

  text    data     bss     dec   filename
  3369      56       0    3425   net/bridge/br_input.o.before
  3458      40       0    3498   net/bridge/br_input.o.after

This allows removal of the "br_should_route_hook" in the next patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>