Do not offload IGMP/MLD messages as it could lead to IGMP/MLD Reports
being unintentionally flooded to Hosts. Instead, let the bridge decide
where to send these IGMP/MLD messages.

Consider the case where the local host is sending out reports in response
to a remote querier like the following:

       mcast-listener-process (IP_ADD_MEMBERSHIP)
          \
          br0
         /   \
      swp1   swp2
        |     |
  QUERIER     SOME-OTHER-HOST

In the above setup, br0 will want to br_forward() reports for
mcast-listener-process's group(s) via swp1 to QUERIER; but since the
source hwdom is 0, the report is eligible for tx offloading, and is
flooded by hardware to both swp1 and swp2, reaching SOME-OTHER-HOST as
well. (Example and illustration provided by Tobias.)

Fixes: 472111920f1c ("net: bridge: switchdev: allow the TX data plane forwarding to be offloaded")
Signed-off-by: Joseph Huang <redacted>
---
v1: https://lore.kernel.org/netdev/20250701193639.836027-1-Joseph.Huang@garmin.com/ (local)
v2: https://lore.kernel.org/netdev/20250714150101.1168368-1-Joseph.Huang@garmin.com/ (local)
    Updated commit message.
v3: Return early if it's an IGMP/MLD packet.
---
 net/bridge/br_switchdev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/bridge/br_switchdev.c b/net/bridge/br_switchdev.c
index 95d7355a0407..9a910cf0256e 100644
--- a/net/bridge/br_switchdev.c
+++ b/net/bridge/br_switchdev.c

@@ -17,6 +17,9 @@ static bool nbp_switchdev_can_offload_tx_fwd(const struct net_bridge_port *p,
 	if (!static_branch_unlikely(&br_switchdev_tx_fwd_offload))
 		return false;
 
+	if (br_multicast_igmp_type(skb))
+		return false;
+
 	return (p->flags & BR_TX_FWD_OFFLOAD) &&
 	       (p->hwdom != BR_INPUT_SKB_CB(skb)->src_hwdom);
 }