[PATCH 0/9] net: bridge: reduce multicast checks in fast path

This patchset introduces new state variables to combine and reduce the
number of checks we would otherwise perform on every multicast packet
in fast/data path.
  
The second reason for introducing these new, internal multicast active
variables is to later propagate a safety mechanism which was introduced
in b00589af3b04 ("bridge: disable snooping if there is no querier") to
switchdev/DSA, too. That is to notify switchdev/DSA if multicast
snooping can safely be applied without potential packet loss.
    
Regards, Linus

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# Changelog

Changelog to / follow-up of: [PATCH net-next 0/5] net: bridge: propagate safe mcast snooping to switchdev + DSA
-> https://lkml.org/lkml/2025/5/22/1413

* removed the switchdev/DSA changes for now
* splitting "[PATCH net-next 1/5] net: bridge: mcast: explicitly track active state"
  into:
  * net: bridge: mcast: track active state, IGMP/MLD querier appearance
  * net: bridge: mcast: track active state, foreign IGMP/MLD querier disappearance
  * net: bridge: mcast: track active state, IPv6 address availability
  * net: bridge: mcast: track active state, own MLD querier disappearance
  * net: bridge: mcast: use combined active state in fast/data path
  * net: bridge: mcast: track active state, bridge up/down

* rebased to current net-next/main:
  * from_timer() -> timer_container_of()

* net: bridge: mcast: export ip{4,6}_active state to netlink:
  * changing NLA_U8 to NLA_REJECT to make it read-only

* moved br_multicast_update_active() call from br_ip{4,6}_multicast_query_expired()
  (own querier timer callback) to br_ip{4,6}_multicast_querier_expired()
  (other querier timer callback)
  * even though both should have worked as br_multicast_querier_expired()
    would call br_multicast_start_querier()->...->br_multicast_query_expired(),
    even if the own querier is disabled, but let's use the more direct way

* simplified br_multicast_update_active():
  * no return value for now, don't track if the active state has changed,
    these aren't necessary (yet)
  * removed __br_multicast_update_active() variant as was used to force
    an inactive state in __br_multicast_stop(), instead using an
    netif_running(brmctx->br->dev) check in br_multicast_update_active()
  * replaced br_ip{4,6}_multicast_check_active() with simpler
    br_ip{4,6}_multicast_update_active() and
    br_ip{4,6}_multicast_querier_exists()
  * fixing build errors with CONFIG_IPV6 unset
* simplified br_multicast_toggle_enabled()
  * no return value for now
  * fixes "old used uninitialized" issue

* removed const from __br_multicast_querier_exists()'s "bool is_ipv6"
* replaced "struct ethhdr *eth" in br_multicast_{snooping,querier}_active()
  with direct ethernet protocol integer attributes
* added a few comments in br_multicast_update_active() calling functions