netfilter: br_netfilter: do not skip all hooks with 0 priority

When br_netfilter module is loaded, skbs may be diverted to the
ipv4/ipv6 hooks, just like as if we were routing.

Unfortunately, bridge filter hooks with priority 0 may be skipped
in this case.

Example:
1. an nftables bridge ruleset is loaded, with a prerouting
   hook that has priority 0.
2. interface is added to the bridge.
3. no tcp packet is ever seen by the bridge prerouting hook.
4. flush the ruleset
5. load the bridge ruleset again.
6. tcp packets are processed as expected.

After 1) the only registered hook is the bridge prerouting hook, but its
not called yet because the bridge hasn't been brought up yet.

After 2), hook order is:
   0 br_nf_pre_routing // br_netfilter internal hook
   0 chain bridge f prerouting // nftables bridge ruleset

The packet is diverted to br_nf_pre_routing.
If call-iptables is off, the nftables bridge ruleset is called as expected.

But if its enabled, br_nf_hook_thresh() will skip it because it assumes
that all 0-priority hooks had been called previously in bridge context.

To avoid this, check for the br_nf_pre_routing hook itself, we need to
resume directly after it, even if this hook has a priority of 0.

Unfortunately, this still results in different packet flow.
With this fix, the eval order after in 3) is:
1. br_nf_pre_routing
2. ip(6)tables (if enabled)
3. nftables bridge

but after 5 its the much saner:
1. nftables bridge
2. br_nf_pre_routing
3. ip(6)tables (if enabled)

Unfortunately I don't see a solution here:
It would be possible to move br_nf_pre_routing to a higher priority
so that it will be called later in the pipeline, but this also impacts
ebtables evaluation order, and would still result in this very ordering
problem for all nftables-bridge hooks with the same priority as the
br_nf_pre_routing one.

Searching back through the git history I don't think this has
ever behaved in any other way, hence, no fixes-tag.

Reported-by: Radim Hrazdil <rhrazdil@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>