From: Nikolay Aleksandrov <redacted>
Date: Sun,  2 Aug 2020 15:50:39 +0300

We need to clear all of the bridge private skb variables as they can be
stale due to the packet having skb->cb initialized earlier and then
transmitted through the bridge device. Similar memset is already done on
bridge's input. We've seen cases where proxyarp_replied was 1 on routed
multicast packets transmitted through the bridge to ports with neigh
suppress and were getting dropped. Same thing can in theory happen with the
port isolation bit as well. We clear only the struct part after the bridge
pointer (currently 8 bytes) since the pointer is always set later.
We can now remove the redundant zeroing of frag_max_size.
Also add a BUILD_BUG_ON to make sure we catch any movement of the bridge
dev pointer.

Fixes: 821f1b21cabb ("bridge: add new BR_NEIGH_SUPPRESS port flag to suppress arp and nd flood")
Signed-off-by: Nikolay Aleksandrov <redacted>

Nikolay, I applied v1 already as I'm not at all against the full clear.