Looking at this code in more detail, I see a slew of problems.

First the list of destinations isn't really a list. The default one is still
embedded in the fdb entry. This means you can't change it safely.

Also the notification via netlink only sends back a single destination
value. 

And the lack of locking on the open coded link list means it is not safe since
the forwarding table is used with RCU. In order to be safe, proper RCU
barriers would be needed or better yet convert to list_rcu..

Overall, I feel guilty for not inspecting this more closely and am surprised
that others did not catch the lack of locking.